WordNet sense 1 (information that makes more forcible or convincing):

reenforcement

WordNet sense 2 ((psychology) a stimulus that strengthens or weakens the behavior that produced it):

reinforcer, reinforcing stimulus

WordNet sense 3 (a device designed to provide additional strength):

strengthener

WordNet sense 4 (an act performed to strengthen approved behavior):

reward

WordNet sense 5 (a military operation (often involving new supplies of men and materiel) to strengthen a military force or aid in the performance of its mission):

support