Jisho

It's short for 全角, meaning full-width, as an easier way of telling which it is (as opposed to looking and figuring out).
Full-width characters occupy the "full" space of a character, while half-width occupies only half (in fixed width fonts, but the terminology extends). In other words, Half-width characters are what we normally type in English, the full-width versions of the roman alphabet occupy as much space as a kanji character
As a comparison:
ＦＵＬＬ
FULL

There are also half-width versions of kana characters: か→ｶ

A small clarification: there are half-width versions of katakana characters, no hiragana.

Ah, thank you! If I expand the autocorrect
options I can see the 半角 options too. E.g., コーヒー [全] or ｺｰﾋｰ [半].