Deviation means straying or moving away from what's normal, expected, or planned. When a pilot makes a deviation from the flight path to avoid a storm, the plane temporarily goes off its planned route. When a scientist notices a deviation in their experiment's results, something unexpected has happened that doesn't match the pattern.

The word often appears when people are measuring, planning, or following rules. A deviation from the recipe might mean adding extra sugar or leaving out an ingredient. A deviation from your usual bedtime routine might mean staying up to finish an exciting book. In statistics, scientists measure how much individual results deviate from the average to understand whether differences matter or are just random variation.

Deviation doesn't always mean something bad. Sometimes deviating from the plan leads to wonderful discoveries. Alexander Fleming's discovery of penicillin came from a deviation in his experiment: mold accidentally contaminated his bacterial cultures, and instead of throwing them away, he investigated why the bacteria died. But in other situations, like air traffic control or following safety procedures, even small deviations can cause serious problems.