You've asked a good question. The terms are really are referring to the same stuff (molten rock). It is just easier this way. If someone is talking about magma you know they are talking about when it is still underground but if they are talking about lava you know they are talking about after it has erupted.
The word lava comes from the Italian word for avalanche which is based on the Latin word "labes" (a sliding down). We guess that this refers to the front of aa lava flows where clinker's are continuous falling down.
We looked up magma in the fattest dictionary we could find. It said: "...akin to Greek MASSEIN to knead..." We guess it has something to do with kind of looking like gooey bread dough?
Image credit: Steve Mattox, July 1990