Disastrous means causing great damage, harm, or failure. When something goes disastrously wrong, the results are terrible and can affect many people. A disastrous fire might destroy dozens of homes. A disastrous decision in a soccer game could cost your team the championship.

A disastrous science experiment might explode and ruin the lab. A disastrous school play might involve forgotten lines, collapsed scenery, and a fire alarm going off. Notice that disastrous suggests more than just “bad.” A low grade is disappointing, but a disastrous grade would be one that threatens whether you pass the class at all.

People also use the word more lightly: “My first attempt at baking was disastrous” might just mean the cookies burned and the kitchen got messy. Context tells you whether someone means truly catastrophic or just really, really bad.