approximately; about (indicating an approximate amount, time, or degree)
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.
料金は約5,000円です。
The fee is approximately 5,000 yen.
a promise; an appointment; an engagement (noun). This standalone noun use is somewhat archaic—modern Japanese usually uses 約束.
彼との約はまだ有効だと考えている。
I consider the appointment with him still valid.
この約を破るわけにはいかない（※日常では「約束」を使うことが多い）。
I cannot break this promise (note: in daily use people usually say '約束').
shortening; reduction; simplification (a noun indicating that something is abbreviated or simplified)
長い説明は約して伝えます。
I'll convey the long explanation in a shortened form.
contraction (in phonetics); shortening of sounds—used in linguistics
この方言では語末の母音が約して消えることがある（約音）。
In this dialect, final vowels may be contracted and disappear (a contraction).