Both mermaid and siren are nouns, and both describe a female creature, but that is where the commonalities end, despite being frequently confused for one another in early use. During the medieval time period, the confusion between mermaid and siren began to occur as the original attributes of siren from classical mythology as a deceitful woman who sings sweetly were shared with mermaid. This musical link of temptation ties them together today still.
According to the OED, both terms came into use around the same time. Mermaid, an original English word, was formed by compounding MERE + MAID, similar to merewīf, or water witch (OED, 2019). A mermaid is “an imaginary, partly human sea creature with the head and trunk of a woman and the tail of a fish or cetacean (OED, 2019). On the other hand, Siren, is originally Greek, and first appeared in the Odyssey. It comes to English via Latin; Sīrēn. In its Latin use, it meant “an imaginary species of serpent,” a meaning that is now obsolete. The most common meaning of siren comes from classical mythology and refers to “one of several fabulous monsters, part woman, part bird, who were supposed to lure sailors to destruction by their enchanting singing” (OED, 2019). This is the usage that was confused frequently with mermaid, which has an additional rarely used form of meaning from the late 16th and 17th centuries: “A woman possessing the attributes of a mermaid; a woman who sings sweetly, or who charms, allures or deceives; a siren… also… a prostitute” (OED, 2019). This idea that both sea creatures sing beautifully has continued importance in its meaning.
According to the Greek poets, the sirens were able to “entice seamen by the sweetness of their song to such a degree that the listeners forgot all and died of hunger. The word thus came to be applied to any dangerous, alluring woman” (Brewers, 2012). Shakespeare famously made this connection in the Comedy of Errors, when he wrote “Oh traine me not sweet Mermaide with thy note, To drowne me in.. teares: Sing Siren for thy selfe” (W.Shakespeare, OED), referring to that deceptive prostitute. Likewise, the entry for Siren has several references that include both pleasure and destruction together, such as another from Shakespeare, in Titus Andronicus: “This Queene… This Syren that will charme Romes Saturnine, And see his shipwracke” (W.Shakespeare, OED). Even if it was not her singing that destroyed you, it was her wanton deception.
The connection between mermaid and siren as a female creature that sang beautifully was made mainly through Hans Christian Anderson’s Little Mermaid. Here we find a mermaid who exchanges her tongue (and voice) for a pair of human legs (Blakemore, 2001). This idea of a demonic and seductive creature who pursues transcendence through the power of love created a romantic overture to the meaning of the word (Zipes, 2002). Likely the most common example of a mermaid that is known today is from Disney’s The Little Mermaid, in which the mermaid strikes the “standard deal”: “Ariel gives up her beautiful voice in exchange for legs,” although a Russian show, Rousalochka in the 70s featured Andersen presenting “a new side to the fabled beauty of mermaids’ voices—it is their siren singing, rather than a storm, that causes the prince’s ship to be dashed against rocks in the first place—before dwelling on the particular mermaid who saves the price” (Zipes, 2002). This ties back to its use in maritime folklore, where a mermaid was believed to be “a harbinger of storms and sea disasters, or a mariner’s protectress” (Haase, 2007).
During “the later Middle Ages, the fish-woman mermaid supplanted the bird-woman siren as the creature believed to lure sailors astray, although in many languages words based on ‘siren’ continued to be used for the fish-woman” (Blakemore, 2001). It is likely this shift in type of female singing danger was due to the ability of mariners to travel the open sea, where mermaids live, instead of along the coastal rocks where sirens were thought to perch. These mermaids were believed to be able to raise and calm storms at will and could trap men with questions and riddles (Blakemore, 2001). This added idea that a mermaid would want to trap a man comes from the 19th century, after the appearance of The Little Mermaid. With a renewed interest in the sexual availability of a mermaid, it is believed a lot of the allure she holds is that “she is sexually unattainable except through death… a man who marries a mermaid can never leave her” (Blakemore, 2001), giving her the same level of danger as a siren.
The association of Sirens with lust or temptation existed from its usage in classic mythology, but never went away. Dante’s dream features a Siren in Purgatorio, as did Chaucer, Shakespeare, and was a popular allegory in medieval English sermons (Brumble, 1998). Its eventual identification as a mermaid is also traced back to medieval era bestiaries, likely because in many Romance languages siren became the word for mermaid (South, 1987). Because of that, modern examples of the usage of siren are harder to come by, but there are some. In literature, examples are of its use in Kurt Vonnegut’s novel The Sirens of Titan, and Pier’s Anthony’s The Source of Magic (South, 1987). The most current use of siren in film is the TV series, Siren, which premiered in March of 2018 (Siren, 2018), and depicts a female siren who communicates only through ethereal siren song, and who kills men who wrong her.
The use of siren and mermaid have been entwined since the middle ages due to increased ability of travel, its use in bestiaries and literature. Despite having very different original meanings, their proximity to dangerous parts of the sea was the largest contributing factor in their synonymity of meaning. I would posit that the largest difference between their meanings in current usage is that mermaid is a being that can gain a soul through love, while siren is a female being who embodies temptation.
 All further references are taken from the OED for both mermaid and siren, unless noted.
 In French, the word for mermaid is siren, somewhat ironic since that is the birthplace of The Little Mermaid.
 Ironically, she is described as a mermaid in the description of the show.
Blakemore, C., & Jennett, S. (2001). mermaid. Oxford Companion to the Body. : Oxford University Press. Retrieved 14 Oct. 2019, from https://www-oxfordreference-com.ezproxy.gvsu.edu/view/10.1093/acref/9780198524038.001.0001/acref-9780198524038-e-613.
Brumble, H. David. “SIRENS.” Classical Myths and Legends in the Middle Ages and Renaissance: A Dictionary of Allegorical Meanings. Santa Barbara, CA: Greenwood Press, 1998. World Folklore and Folklife. ABC-CLIO. 14 Oct 2019. http://folklore.greenwood.com.ezproxy.gvsu.edu/wff.aspx?k=6&x=GR9451&=p=GR9451-2979&bc=.
Dent, S. (Ed.). (2012). Parthenope. Brewer’s Dictionary of Phrase & Fable. : Chambers Harrap Publishers. Retrieved 14 Oct. 2019, from https://www-oxfordreference-com.ezproxy.gvsu.edu/view/10.1093/acref/9780199990009.001.0001/acref-9780199990009-e-7912.
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Haase, Donald. “Encyclopedia of Folklore and Fairy Tales: Mermaid.” The Greenwood Encyclopedia of Folktales and Fairy Tales. Santa Barbara, CA: Greenwood Press, 2007. World Folklore and Folklife. ABC-CLIO. 14 Oct 2019. http://folklore.greenwood.com.ezproxy.gvsu.edu/wff.aspx?k=6&x=GR3443&=p=GR3443-2877&bc=.
“mermaid, n.”. OED Online. September 2019. Oxford University Press. https://www-oed-com.ezproxy.gvsu.edu/view/Entry/116826?redirectedFrom=mermaid& (accessed October 14, 2019).
“siren, n.”. OED Online. September 2019. Oxford University Press. https://www-oed-com.ezproxy.gvsu.edu/view/Entry/180372?isAdvanced=false&result=1&rskey=Xp97jf& (accessed October 14, 2019).
Siren. (2018, March 29). Retrieved from https://www.imdb.com/title/tt5615700/.
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South, Malcolm. “Sirens.” Mythical and Fabulous Creatures: A Source Book and Research Guide. Santa Barbara, CA: Greenwood Press, 1987. World Folklore and Folklife. ABC-CLIO. 14 Oct 2019. http://folklore.greenwood.com.ezproxy.gvsu.edu/wff.aspx?k=6&x=SMY&=p=SMY-1146&bc=.
Zipes, J. (2002). ‘Little Mermaid, The’. In The Oxford Companion to Fairy Tales. : Oxford University Press. Retrieved 14 Oct. 2019, from https://www-oxfordreference-com.ezproxy.gvsu.edu/view/10.1093/acref/9780198605096.001.0001/acref-9780198605096-e-466.
Zipes, J. (2002). mermaid fairy-tale films. In The Oxford Companion to Fairy Tales. : Oxford University Press. Retrieved 14 Oct. 2019, from https://www-oxfordreference-com.ezproxy.gvsu.edu/view/10.1093/acref/9780198605096.001.0001/acref-9780198605096-e-508.