It is any surface-level or unanalyzed sound of a language, a smallest identifiable unit occurring inside a stream of speech.
Phones are the segments of speech that possesses distinct physical or perceptual properties, regardless of whether the exact sound is critical to the meanings of words.
Whereas a phone is a concrete sound used across various spoken languages, a phoneme is more abstract and narrowly defined: any class of phones that the users of a particular language nevertheless perceive as a single basic sound, a single unit, and that distinguishes words from other words.
However, the difference between the [ɕ] ⓘ sound in some dialects' pronunciation of sheet and the [ʃ] ⓘ in shack ([ɕit] versus [ʃæk] in the IPA) never affects the meaning or identity of a word in English.
In the examples above the characters enclosed in square brackets: "pʰ" and "p" are IPA representations of phones.