How is 指 used, what is it's role in definitions such as this (from Wikipedia):
In my understanding, 指 (zhi3) means "refers to". Sometimes it is interchangeable with 表示 (biao3shi4) or 表达 (biao3da2), which mean "expresses". Just like in an English dictionary, you might see a word with a definition that says X refers to Y or X is used to express Y. These words are used in a basically parallel way.