To widen means to make something bigger or more extensive.
When we talk about widening something, we usually mean making it wider or larger. Imagine a street that's always congested with cars - if they decide to widen it, they'll make it broader so there's more space for vehicles to move through. In everyday life, people might use the phrase "widen their horizons" to describe learning new things and becoming more open-minded. It can also mean making a gap or opening bigger - think of widening your eyes in surprise or widening a doorway to let more light in.
To become wide or wider.
Example: His eyes widened as her negligee fell to the floor.
