A note is a very short piece of writing. You might write a note to your sister on the back of your grocery list, or jot down a quick note reminding yourself to call your grandmother later.
You might slip a love note under a door, or leave your cat sitter a note explaining how much to feed Fluffy. Other notes are taken during lectures or written in notebooks by journalists during interviews. You can also make a mental note, paying particular attention to something in the hopes that you'll remember it later. The Old French noter, "indicate or write down," comes from the Latin root notare, "to mark or note," from nota, "mark."