To twitter is to make a sound like a bird's trill or chirp. One of the nicest things about spring is the sound of birds as they twitter in the trees. Tweet tweet!
When birds twitter, they make a staccato, musical chirping sound. You can call this sound itself a twitter too. Sometimes the word is used to describe a similar sound made by chattering humans. In the fourteenth century, the word was twiteren, developed from an imitation of the sound that twittering birds make. Twitter is also a social media website founded in 2006, on which people post 140-character bursts of information called tweets. Wonder where they got that idea.