The adjective, congruent fits when two shapes are the same in shape and size. If you lay two congruent triangles on each other, they would match up exactly.
Congruent comes from the Latin verb congruere "to come together, correspond with." Figuratively, the word describes something that is similar in character or type. Are your actions congruent with your values? If a friend says something outrageous that you don't want to agree with but don't want to disagree with either, say that your friend's idea is congruent with what you think. That way you can agree with him but change your mind later if you have to.