Allophony

Allophony is the phenomenon that a phoneme has different (context-dependent) variants. The different variants are called allophones.

Example
In English [th] in tin, [?t] in cat and [t] in stop are allophones of the phoneme /t/.

Comments
Syllable-initially the phoneme is pronounced differently than in medial or in final position. The differences between the allophones can be expressed by phonological rules.

Link
Utrecht Lexicon of Linguistics