Last year we discussed maror; now let's talk about a vegetable that can be used for maror - חזרת hazeret (or chazeret).
The Mishna in Pesachim (2:6) lists a number of vegetables that can be used to fulfill one's obligation on the night of Pesach - and includes in the list hazeret. The gemara there (39a) translates hazeret from Hebrew into Aramaic - chasa חסא - lettuce. Rashi also identifies hazeret as lettuce - using the French word laitugue, which is related to the English word lettuce.
So how did hazeret come to be used for horseradish in Modern Hebrew? According to Dr. Yehuda Feliks here, when the Jews moved further East in Europe, lettuce was no longer available for Pesach. The Chacham Tzvi (1656-1718) wrote that the Jews of Ashkenaz and Poland would use tamcha תמכא for maror, because they couldn't find hazeret. Tamcha is one of the other vegetables listed in the mishna as acceptable, and the Tosfot Yom Tov (1579-1654) identifies it with horseradish (a relatively late identification- the Rambam identifies it with a type of chicory and Rashi translates it as Marrubium vulgare - horehound). Horseradish is known as chrain in Yiddish - the origin of the word is unknown, but it has many cognates in European languages. However, over time the word hazeret became associated with horseradish instead of tamcha. This entered Modern Hebrew as well, despite the availability of lettuce. However, lettuce seems like a better fit, because horseradish is not as bitter as it is sharp, and the Yerushalmi on Pesachim says that hazeret starts out sweet and ends bitter (like the Jews' experience in Egypt) - a much better description of lettuce than horseradish. For more about the development of the use of horseradish for maror, read this interesting article by Ari Zivitofsky (thanks to Parshablog for the link).
Interestingly, none of the sources I looked at - Jastrow, Ben Yehuda, Klein - offer an etymology for hazeret, despite the easily identifiable root of חזר (to return). I do suspect, however, that there are many drashot out there...