The act of doing something alone, though not necessarily while in the air.
Solo is defined as something done alone without accompaniment. If you go to a party alone, this is an example of when you go solo.
Solo flight means you are in the aircraft all by yourself. There is no instructor in the aircraft. He or she is waiting patiently, or nervously, as the case may be, at the flying school or in the Tower, for your safe return.