Readers are not stupid. If they do not know what a word is, they can infer its meaning from context or (gasp) look it up.
Note that this post is also related to my post on Research.
Think of trusting your reader as an aspect of being true to your character (not you, personally, the ones in your book). Your character will know stuff you don't, and also stuff you (or the reader) don't necessarily need to know. That's what makes a well-rounded character -- if there weren't this idea that he has a depth beyond that which is presented, he won't seem realistic.
For example, toward the end of NINJAS, I have a scene in which Elias comes home, his father is sawing away at the cello, and Elias observes that something to the effect that "Beastmaster VII ran downstairs and hid under the Flemish double harpsichord."
Now, a harpsichord is a fairly unusual instrument (these days). A Flemish double harpsichord is even more unusual. However, Elias doesn't think anything else about it, does not explain why on earth the family would own a Flemish double harpsichord, or even tell the reader what a Flemish double harpsichord is. He just goes upstairs to confront his father.
He does not expalin because he would not explain. To him, the Flemish double harpsichord is as much a part of the background as a televison set.
Trust your reader.