What is a long vowel?

The definition of a long vowel really depends on the language you are talking about. In English, a long vowel is usually an accented vowel. So the "a" in decamp is a short vowel, but the "a" in delay is a long vowel. It is very different in other languages, where a long and short vowel might be pronounced exactly the same, but the long vowel is pronouncel longer (you stay on that sound for longer). For example, in Hawaiian, an a might be either short or long anywhere in a word. For example, Ka'anapali has a long a before the apostrophe (which is actually a letter in Hawaiian), and a short a after it.