A flight leg in the air industry is a flight from one airport to another under the same flight number, with the same aircraft, and no stops in between. The term is typically used to define a part of a complex (also called multi-leg) itinerary that consists of multiple flight legs meaning that layovers and/or changes of aircraft are involved in such a journey.
For example, a direct, non-stop flight from New York to Boston includes only one flight leg. A flight from New York to Paris with a layover in London includes two legs, regardless of whether it’s the same aircraft, different planes of the same carrier, or even different carriers.
A flight leg is often used interchangeably with the term flight segment, but there’s a difference between the two. A flight segment is also a trip from origin to destination airport, but it can have a stop, for example, for refueling, when passengers don’t have to leave the plane. In case there’s an intermediate stop, a flight segment consists of two legs.