A prefix meaning 'un-' or 'non-'; attached to words (mainly Sino-Japanese compounds) to indicate negation or absence (e.g., 不可能, 不安).
その計画は現時点では不可能だ。
That plan is impossible at the moment.
不安を感じるときは誰かに相談したほうがいい。
If you feel anxious, it's better to talk to someone.
漢字の前に『不』を付けて否定の意味を作ることが多い。
It's common to put '不' in front of a kanji to form a negative meaning.