Definitions

1. noun

a substance or material used to stiffen or stiffening a material

“The fabric required a stiffener to give it the right texture for the garment.”

2. verb

to make or become stiff or more rigid

“The exercise stiffened her muscles after a long period of inactivity.”

3. verb (transitive)

to make (something) more rigid or stiff

“The heat stiffened the wood, making it harder to work with.”