A mora is the amount of time it takes to say one part of a syllable. It is a linguistic term and is a part of phonology that aims to study sound formation. Each mora takes the same amount of time to pronounce; longer sounds are represented by two or even three moras as they take a longer amount of time to pronounce. Not all languages have even rules regarding what is a mora and what is not. In languages where syllable length is important, the mora can play an important role in linguistic forms like poetry.
In phonology, a syllable is broken down into one, two, or three units. These units determine the number of moras and, therefore, the weight of the syllable. Syllables can be made of up to three parts: the nucleus, the onset, and the coda. The nucleus is the main sound unit, and the onset and coda are consonants that come before and after the nucleus; the coda may also be a vowel.
The onset of a syllable does not count towards that syllable's weight. The nucleus may consist of a short vowel or consonant that counts as one mora. A long vowel or consonant in the nucleus counts as two moras.
The lengths of vowels and consonants varies considerably by language. In many languages, consonants are only short. In Slovak, however, consonants can be either short or long. In Malayalam, a consonant does not contribute any weight to the syllable and counts as zero moras.
Codas do not automatically add length to a language's syllable weight. In Japanese, all codas count as one mora. The coda also bears weight in Hindi and Malayalam, but not in Irish. In English, the coda only counts as one if it is stressed.
A syllable that contains one mora is called monomoraic and is classed as a light syllable. When there are two moras in the syllable, it is bimoraic and is a heavy syllable. The majority of languages have either monomoraic or bimoraic syllables, but some languages like Old English and Farsi have trimoraic or super heavy syllables with three moras.
In addition to their role in everyday speech, moras are important to poetry. Classical languages such as Latin, Greek, and Sanskrit are mora-based languages. The poetry of these languages tends to use dactylic meter with combinations of long and short syllables providing rhythm.
In Japanese, the mora is called a haku. The role of the haku has been studied extensively in Japanese. It has also been posited by linguists that the Japanese poetry form of haiku should not be thought of as 17 syllables, as it often is in English, but as 17 moras because this more closely matches Japanese.