In spoken language, a chroneme is a basic, theoretical unit of sound that can distinguish words by duration only of a vowel or consonant. Most languages have differences in length of vowels or consonants, but not many use lenght alone to distinguish words.

Two words with different meaning that are spoken exactly the same except for length of one segment are called a minimal pair. English does not have such minimal pairs and is theoretically said to have only one chroneme.

Classic Latin, Italian, Hungarian and Spanish have distinctive length in consonants. E.g. in Italian

pina  pine
pinna fin

Classic Latin, German, Hungarian and Thai have distinctive length in vowels length. E.g. in Thai:

khâo  (เข้ว) short: enter
khâ:o (ข้าว) long:  rice

The International phonetic alphabet (IPA) denotes length doubling the letter or by diacritics above or after the letters: