A pendant is a jewel, stone, or small ornament that hangs from a thin chain you wear around your neck. The hanging is the point — the word comes from old French for 'hanging'. A necklace can be just a chain; once something dangles from it, that dangling piece is the pendant. It is an ordinary jewellery and shopping word, not slang and not stiff. Though actually, older and more technical English also used pendant for other hanging things, like a hanging light or an ornament in a church roof; what learners meet first, and what this sense is, is the necklace charm. Do not mix it with pennant (a small flag) or pedant (a fussy know-it-all).