On the fly can be used to describe doing something quickly or making a change without planning. Some synonyms for on the fly include ad hoc, impromptu, spontaneous, unplanned, extemporaneous, and improvisational. These terms all include the idea of doing something without prior preparation or thought. Other synonyms for on the fly can also include in real-time, on the spot, and instantly. These words all convey the sense of acting quickly or reacting immediately to a situation. Using synonyms for on the fly can help writers and speakers vary their language and keep their communication fresh and engaging.