Unfavorable means not helpful, not good, or working against you. When conditions are unfavorable for something, they make success harder or less likely.

Weather forecasters might predict unfavorable conditions for a planned outdoor field trip: heavy rain, strong winds, or extreme cold that would make the day miserable or unsafe. A scientist running an experiment might get unfavorable results, meaning the data doesn't support her hypothesis. A book reviewer might write an unfavorable review, explaining why they didn't think the book was very good.

The word often describes circumstances or judgments. Unfavorable weather hurts a farmer's crops. An unfavorable comparison makes one thing look worse when placed next to another. If you're trying to convince your parents to get a puppy, discovering that your dad is allergic to dogs would be an unfavorable development.

The opposite is favorable, meaning helpful or positive. Notice that unfavorable doesn't necessarily mean terrible or disastrous. It simply means the situation isn't working in your favor, like trying to start a campfire when everything's damp: not impossible, but the conditions aren't helping you.