English:
A short vowel is the counterpart of a long vowel. So the “a” in gate is long while the “a” in bat is short.
Greek:
- ε and ο are always short.
- η and ω are always long.
- α and ι can be either.
Hebrew:
In Hebrew, patah, segol, hiriq, qamet-hatuf, and quibbuts are all short vowels. See this chart.