"What does this mean" plugin needed

Hey there,

Im looking for a plugin but I dont know how to word the search to find it.
What Im looking for basically is to enter a definition for some words or terms I use on my blog that regular people might not know and that will appear when you place your mouse on the word.

Hope I made myself clear. I guess is a very popular plugin but dont know the name and would love to have it.

Thanks in advance!