Trimming and pruning are different actions.
Put crudely, you trim with one of these
You prune with one of these
Trimming is the action of keeping a plant in a desired shape and removing excess, untidy growth. Pruning is the removal of dead growth or the encouragement of new growth in a certain area. Pruning is a more targeted action than trimming.
My impression is that trimming is done on softer, newer growth, and pruning is done on stiffer, older growth, with bark, or branches, and stuff.
You'll note that you don't prune your lawn.
Trim is sometimes used as a synonym for prune (correctly or not), but the reverse doesn't seem true: Trimming might refer to a small amount of pruning, though.
Weighing in a bit late here, but does any one use Symphyotricum instead of Aster? Hylotelephium instead of Sedum? Lamprocapnos spectabilis instead of Dicentra spectabilis? No. Just because some taxonomist has to write a paper doesn't mean that we need to follow blindly along. We need, IMO, to follow the Horticulture trade in these instances, or we can ...