Fetching details from the dictionary...

soliloquy; monologue; speaking to oneself
祖母は台所で一人言を言いながら料理をする。
My grandmother cooks while talking to herself in the kitchen.
一人言が増えたのは一人暮らしのせいかもしれない。
The increase in talking to myself might be because I live alone.
一人言のつもりが隣の人に聞こえていた。
What I meant as talking to myself was overheard by the person next to me.