strengthen verb [I/T] (BECOME EFFECTIVE) to become more effective, or to make something more effective: [ T ] Somehow his illness strengthened their marriage and brought them closer together. [ T ] Parents can strengthen ties with their children.

English Language Learners Definition of strengthen. : to make (someone or something) stronger, more forceful, more effective, etc. : to become stronger, more forceful, more effective, etc. of money : to increase in value when compared to money from other countries. See the full definition for strengthen in the English Language Learners Dictionary.