A Megadere is basically obsessed with their love interest, but not in the insane way a yandere is. They constantly fangirl/fanboy over their love interest whenever they can, and might stalk them on occasion. Despite their obsessiveness, megadere characters don't take their obsessive love to an extent where it causes them to commit violent acts or manipulate their love interest, unlike a yandere.

The word is mysterious, but it probably comes from the Japanese word Megane 眼鏡(めがね) meaning "glasses", or the word "mega", comes from the English word "mega" which means big or huge.

source: https://the-dere-types.fandom.com/wiki/Megadere