Allophone

a variant of a particular phoneme in a language.

Example in English: / p / can be aspirated as in pin: [phɪn] or unaspirated (plain) as in spin [spɪn].

Square brackets [ ] are used with phonetic notation (when the differences between allophones are included. Slashes / / are used when only phonemes are written.

Swan, Michael. "Seven bad reasons for teaching grammar -- and two good reasons for teaching some".