A once-over is a quick but thorough look at something or someone, checking to see if everything seems okay or noticing the important details. When your mom gives your room a once-over before guests arrive, she's scanning quickly to make sure nothing looks too messy. When a teacher gives your essay a once-over, she's reading through it rapidly to get a general sense of your work before grading it more carefully.

The phrase suggests speed combined with alertness. You're scanning for what matters without studying every tiny detail. A coach might give new players a once-over to quickly assess their skills. A detective might give a crime scene a once-over to spot anything obviously important before examining everything methodically.

Sometimes people use once-over when talking about someone noticing another person's appearance, like when you're dressed up for a school performance and your friend gives you a once-over and says, “You look great!” The expression captures that specific kind of quick, assessing look we all recognize.