Mora (plural moras or morae) is a unit of sound used in phonology that determines syllable weight (which in turn determines stress) in some languages. Like many technical linguistics terms, the exact definition of mora is debated. The term, meaning "delay", comes from Latin.

In general, moras are formed as follows:

1. A syllable onset (the first consonant(s) of the syllable) does not represent any mora.
2. The syllable nucleus represents one mora in the case of a short vowel, and two moras in the case of a long vowel or diphthong. Consonants serving as syllable nuclei also represent one mora if short and two if long. (Slovak is an example of a language that has both long and short consonantal nuclei.)
3. In some languages (for example, Japanese), the coda represents one mora, and in others (for example, Irish) it does not. In English, it is clear that the codas of stressed syllables represent a mora (thus, the word cat is bimoraic), but it is not clear whether the codas of unstressed syllables do (the second syllable of the word rabbit might be monomoraic).
4. In some languages, a syllable with a long vowel or diphthong in the nucleus and one or more consonants in the coda is said to be trimoraic.

In general, monomoraic syllables are said to be light, bimoraic syllables are said to be heavy, and trimoraic syllables (in languages that have them) are said to be superheavy. Most linguists believe that no language uses syllables containing four or more moras.

Japanese is a language famous for its moraic qualities. Most dialects including the standard use moras as the basis of the sound system rather than syllables. For example, haikus in Japanese do not follow the pattern 5 syllables/7 syllables/5 syllables, as commonly believed, but rather the pattern 5 moras/7 moras/5 moras.