This is the 1212th most frequent Japanese word.
非
"非" means "non-, un-, dis-, or negative."
The character 非 is used as part of the word '非常に' (hijōni), meaning 'extremely' or 'very much', indicating a degree or intensity.
そのアイデアは非常に重要です。
That idea is extremely important.
Here, 非 is used as a prefix in '非論理的' (hironriteki), meaning 'illogical', to denote the negation or absence of logic.
彼の行動は非論理的でした。
His behavior was illogical.
The prefix 非 is part of '非正規' (hiseki), meaning 'irregular' or 'off-standard', used to describe a method that doesn't follow standard norms.