Gusset (noun)

A piece of fabric inserted into a garment to strengthen or expand a seam or part of the garment.

Origin:

Early 16th century: probably from french goujure 'triangular piece', based on latin gustus 'taste'. the original sense was 'a triangular piece of material used in reinforcing clothing'; the current sense dates from the late 19th century.

Examples:

1. The gusset on the side of my jeans ripped open.
2. I had to sew in a gusset to make the dress fit me better.
3. The gusset in my backpack helped to keep its shape.
4. The gusset on the bottom of the tent added stability in windy conditions.
5. The gusset of the shirt was made from a different material to enhance durability.