A phoneme is the smallest unit of sound in a language that can change the meaning of a word. Think of phonemes as the building blocks of spoken words, like how letters are the building blocks of written words (though phonemes and letters don't always match up perfectly).

For example, the words “cat” and “bat” differ by just one phoneme: the /k/ sound versus the /b/ sound. That tiny difference in sound creates two completely different words with different meanings. Similarly, “ship” and “shop” differ only in their middle vowel sound, but that single phoneme changes everything.

English has about 44 phonemes, even though our alphabet has only 26 letters. Some phonemes are represented by letter combinations: the “sh” in “ship” is one phoneme, not two separate sounds. The “th” in “think” is also a single phoneme. Meanwhile, the letter “x” actually represents two phonemes together: /k/ and /s/.

When linguists write phonemes, they put them between slashes, like /b/ or /k/, to show they're talking about sounds rather than letters. Understanding phonemes helps explain why English spelling can be tricky: the same phoneme might be spelled different ways (like the /f/ sound in “phone” and “fun”), and the same letter might represent different phonemes (like the “c” in “cat” versus “city”).