A phoneme is a single speech sound, especially one that distinguishes one word from another, like, for instance, the final sounds of hip, hit, hill and hid.
Academics quibble a bit about phonemes because sometimes sounds that people think are the same aren't actually identical (like, if you're a native English speaker, the k sounds in cool and keep).
It follows that sound that's a phoneme in one person's language might split into two or more phonemes in another: small children, for example, often go through a stage of not distinguishing between r and w, or y and l.
Because of this, a phoneme is defined as the smallest unit of sound that can make a difference to meaning in a particular language.
But anyway, what about the phoneme email in my junk folder?
Well, luckily, just before I opened it I realised it should really have read phone me...
Thing To Consider Today: phoneme. This word comes from the Greek phōnēma, sound or speech.