1Approximately; about — placed before a number to indicate it is not exact (e.g. 約一時間 'about an hour', 約50人 'about 50 people').
会議は約一時間で終わった。
The meeting finished in about one hour.
電車は約10分遅れて到着した。
The train arrived about ten minutes late.
参加者は約200人だった。
There were about 200 participants.
駅から会場までは約500メートルです。
It's about 500 meters from the station to the venue.
2A promise or agreement (noun use). In modern everyday Japanese, the bare word 約 is essentially never used this way — 約束 is used instead — but the old sense survives in set literary phrases like 金石の約 ('a pledge as firm as metal and stone').
二人は生涯変わらぬ友情を誓い、金石の約を交わした。
The two of them vowed unwavering lifelong friendship and exchanged an unbreakable pledge.
3Shortening or simplification (noun use) — reducing something to its essential points. In modern Japanese the bare word is rarely used this way on its own; the meaning survives mainly inside compounds like 約分 ('reducing a fraction') and 約数 ('divisor').
4Contraction (in phonetics) — the shortening of sounds; a linguistics term used mainly in grammar reference books.
文法書では、この母音の変化を約と呼ぶことがある。
Grammar books sometimes call this kind of vowel change a 'contraction' (約).