Notation is a system of written marks that stand for ideas, so people who know the system can read them without a long sentence. Music has notation — the notes, rests, and other signs on the lines. Maths has notation, like + or x². Chemistry, dance, and chess have their own systems too. You usually say “musical notation” or “scientific notation” for the whole system (no plural). You can say “a notation” when you mean one way of writing something. It is a school and specialist word, not a chatting word. Though actually, people sometimes say notation when they only mean scribbled notes. Careful speakers keep notation for a real system of signs, and notes for the words you jot down.