by ear

1. Without referencing sheet music or a recording. Refers to the ability to play a piece of music. Wow, George is such a talented piano player! The way he can play pieces by ear after hearing them just once is so impressive! Just because he can play by ear doesn't mean he's a great songwriter. Yeah, I can probably play that guitar part by ear—let me hear it again.

2. By extension, in an improvised or flexible way. Typically used in the phrase "play it by ear." A: "Are we meeting at noon on Saturday?" B: "Around then. I have to run some errands in the morning, so let's play it by ear." My apologies in advance, but I forgot my notes for today's presentation, so I'm afraid I'm going to have to play it by ear a little bit. I know we're supposed to get together tomorrow night, but I haven't been feeling well. Can we play it by ear?