In human language, a phoneme is the smallest unit of speech that distinguishes meaning. Phonemes are not the physical segments themselves, but abstractions of them. An example of a phoneme would be the /t/ found in words like tip, stand, writer, and cat.
In sign languages, the basic movements were formerly called cheremes (or cheiremes), but usage changed to phoneme.
Some linguists (e.g. Roman Jakobson) consider phonemes to be further decomposable into features, such features being the true minimal constituents of language. Features as opposed to phonemes however overlap each other in time. A phoneme could be seen as a contemporaneous bundle of features.
A phoneme can include slightly different sounds or phones. For instance, the p sound in the words pin and spin is pronounced differently, but is still considered the single /p/ phoneme. Two phones that belong to the same phoneme are called allophones. A common test to determine whether two phones are allophones or separate phones relies on finding so-called minimal pairs: words that differ only by the phones in question.