This article may be confusing or unclear to readers. In particular, this article does not explain the subject clearly. (July 2017)
Phonemic contrast refers to a minimal phonetic difference, that is, small differences in speech sounds, that makes a difference in how the sound is perceived by listeners, and can therefore lead to different mental lexical entries for words. For example, whether a sound is voiced or unvoiced (consider /b/ and /p/ in English) matters for how a sound is perceived in many languages, such that changing this phonetic feature can yield a different word (consider bat and pat in English); see Phoneme. Another example in English of a phonemic contrast would be the difference between leak and league; the minimal difference of voicing between [k] and [g] does lead to the two utterances being perceived as different words. On the other hand, an example that is not a phonemic contrast in English is the difference between [sit] and [siːt]. In this case the minimal difference of vowel length is not a contrast in English and so those two forms would be perceived as different pronunciations of the same word seat.