There’s a divergence between American English and British English here; in BrE “capsicum” can mean a bell pepper, but in AmE it only means a chilli pepper. (In neither does “capsicum” mean the substance that gives the hot peppers their hotness; that’s called capsaicin, capsicin, or capsicine.)
In English, capsicum is what makes hot peppers hot. The large peppers that aren’t hot are called peppers, sweet peppers or bell peppers.
There’s a divergence between American English and British English here; in BrE “capsicum” can mean a bell pepper, but in AmE it only means a chilli pepper. (In neither does “capsicum” mean the substance that gives the hot peppers their hotness; that’s called capsaicin, capsicin, or capsicine.)
Capsaicin is the chemical that makes peppers hot; capsicum is a genus.