We'd like to begin this definition with a quote from the erudite Michael Scott:
I am not one to be truffled with. (The Office, "Casual Friday")
And truffle with you, we shall not.
When someone who's not the sharpest tool in the shed uses one word when they mean another, that's a malapropism. Well, it's also a malapropism even if the utterer is a genius. But hey, we're not going to start throwing stones at grass houses, even if we are a suppository of knowledge.
Shakespeare was a huge fan of malapropism. In The Winter's Tale, the clown says "Ay, or else 'twere hard luck, being in so preposterous estate as we are" (5.2). Preposterous? We're thinking he meant prosperous. Those are very different things, Shmoopers. It's very easy to accidentally use a malapropism—it's much more difficult to do it on porpoise.
While we're on the subject, it's fitting that the clown is the speaker of this Shakespearean gem, because malapropisms almost always have comedic effect—whether intentional or not. For public figures, it's almost always not. For authors, it's a toss-up.