To reinforce means to make something stronger or more solid. When workers reinforce a bridge with extra steel beams, they're adding support so it can hold more weight. When you reinforce the knees of your jeans with an iron-on patch, you're strengthening a weak spot so the fabric won't tear.

The word also means to strengthen an idea, belief, or behavior by repeating it or adding support. When a teacher reinforces a lesson by reviewing it the next day, she's helping students remember it better. Parents reinforce good behavior when they consistently praise their children for being helpful or honest. Each time you practice a skill, you reinforce the pathways in your brain, making that skill easier and more automatic.

Reinforcements are additional troops sent to strengthen an army, or any extra help that arrives when someone needs support. If you're building a snow fort and three more friends show up to help, you could call them your reinforcements.

The opposite of reinforce is weaken or undermine. While reinforcing builds up and strengthens, undermining tears down. Reinforcing usually takes consistent effort: one compliment doesn't reinforce confidence, but regular encouragement does.