You know, a pendant group in polymer science means pretty much what you'd expect it to mean, as long as you know what a pendant is! Merriam-Webster defines a pendant as "something suspended: as an ornament (as on a necklace)."

A "pendant" group on a polymer is a small group of atoms (even a small chain sometimes) that hangs off of the main chain (that is, the backbone of the polymer). As you can see from the example, pendant groups are usually part of the repeat unit of the polymer, so there are lots and lots of pendants hanging off of our chain!

Pendant groups are much smaller than the backbone chain. Pendant groups normally have just a few atoms (or maybe even a dozen or so), but the backbone chain usually has hundreds of thousands of atoms.

Here are some examples of pendant groups circled in red:

polypropylene

polyvinylalcohol

polystyrene