Glasses

Easily the most recognizable aspect of Daria Morgendorffer's persona, her manstopper glasses are heavy black eyeglasses with large, round lenses that easily cover a notable portion of her face. (The term was first used by Brother Grimace.)

Her prescription has never been revealed in canon. However, as a number of episodes have shown (most notably "Through a Lens Darkly"), Daria has quite poor eyesight and needs her glasses to function normally, as she kept bumping into classmates when pretending to wear her contacts and had to be guided at times by Jane Lane. Even with her glasses on she has visual problems, once nearly running a dog over while driving because she had poor peripheral vision ("Through a Lens Darkly").

A "future-ego" shot at the end of "Is It College Yet?" shows her as an adult wearing glasses, though more stylish than the manstopper sort, as does her imagined future self in "Write Where It Hurts". Her other imagined future self in "Lane Miserables" gives her larger, square glasses. "Daria Morgendorffer's Greatest Retail Regret", created and set after "College, shows her with the same glasses that she had in the series.

"Through a Lens Darkly" presented the most detailed discussion of Daria's glasses and what they mean to her. Jane Lane said it best in a discussion with her amiga:

Daria: Now my mother's bugging me about contacts again. Jane: Boy, she just doesn't get it, does she? Daria: Nope, she really doesn't. [pause] Get what? Jane: You! The whole Daria Morgendorffer persona. You don't care what people think about your looks. Daria: Of course not. Jane: The glasses are you. They're symbolic of the whole Daria thing. [imitates Daria] "I wear glasses and I'm not going to apologize for it."

Amy Barksdale, often taken to be similar to an older Daria, had no problem with replacing her "manstopper glasses" with contacts, but Daria found contacts too troublesome (and painful) to use. Amy has also kept her glasses and sometimes dons them for visual effect (in "I Don't" to bond with Daria).

Fandom Reactions
Fans of Daria have sometimes commented about their own glasses (or lack of same). Kara Wild (she who gave Quinn Morgendorffer eyeglasses in her Driven Wild Universe cycle) began one such thread on PPMB, which is worth reading for the various reactions people have to glasses. Medea42 initiated another such thread on PPMB, and Brandon League began a like thread more recently, both also worth a look.

Merchandise
An official Daria costume came out in October 2011, complete with a replica of Daria's glasses.