Predominantly means mainly or mostly, but not completely. When something is predominantly one way, that quality clearly stands out even though other qualities might also be present.

If a neighborhood is predominantly residential, most buildings are homes rather than stores or offices, though a few shops might exist here and there. A forest that's predominantly oak trees contains mostly oaks, but you'll still spot some maples, pines, or birches mixed in. When a book is predominantly about adventure, that's the main focus, even if it includes some mystery or friendship elements too.

The word helps describe situations where one characteristic dominates without wiping out everything else. A student whose report card is predominantly A's has earned mostly A's, with perhaps a few B's. A crowd at a concert might be predominantly teenagers, meaning most attendees are teens, though adults and younger kids are there too.

Predominantly is more precise than saying “only” or “all,” because it acknowledges that other elements exist while making clear what the main characteristic is. It's a useful word when you want to describe the overall picture without pretending the exceptions don't exist.