A diadem is a crown, or something on a crown. If you’ve just won the Miss America pageant, reach up your hand — that’s right — that rhinestone encrusted circlet on your head? That’s a diadem.
While diadem is a straightforward word meaning "crown," it can also refer to the jewels or other ornaments on a crown. It also sounds like diamond, which is handy for remembering the meaning, because a crown might well have diamonds on it. Or, the diadem could be the diamond on the crown. Technically, you could have a diadem on a diadem, but it would be confusing to say it that way. It comes from the Greek diadema, which was cloth tied around the head to signify royalty. Not as nice as diamonds though, really!