A variant sound of the same phoneme, or a different pronunciation of a speech sound that doesn't change the meaning of words.
From Greek 'allo-' (other, different) and 'phone' (sound), literally meaning 'different sound'; introduced by linguists in the early 20th century to explain why sounds can vary while meaning stays the same.
The English 'p' sound in 'pit' versus 'spit' sounds different to your ear—that's allophone magic! Your brain treats them as the same sound even though your mouth makes them completely differently depending on context.
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