What signifies an increase in wind direction of over 45 degrees in less than 15 minutes?

An increase in wind direction of over 45 degrees in less than 15 minutes is classified as a wind shift. This term is used to describe a significant change in wind direction, which can occur due to various atmospheric changes such as the passage of a weather front or other meteorological phenomena. Wind shifts are important for pilots and air traffic controllers because they can affect aircraft performance, navigation, and safety during takeoff and landing.

The other options refer to different wind characteristics or conditions. For example, gusts refer to short bursts of increased wind speed rather than a change in direction. Variable wind indicates a wind direction that changes often, but not necessarily in the specified manner or timeframe that a wind shift describes. Crosswind refers to wind that blows across the runway direction, which affects takeoff and landing but does not specifically address the change in direction requirement mentioned in the question. Thus, the correct term for a significant directional change in the wind is indeed a wind shift.