In classical verse, the time required to pronounce a syllable. The Greeks and Romans classified syllables as either 'short' or 'long' and this provided the basis for their metrical patterns. In English verse, however, quantity is important but is not the only consideration - as syllable length is often determined by the position in the line and also by tonic accent.
[n] how much there is of something that you can measure
[n] an adequate or large amount; "he had a quantity of ammunition"
[n] something that has a magnitude and can be represented in mathematical expressions by a constant or a variable