Appreciable means large enough to be noticed, measured, or to matter. If there's an appreciable difference between two things, you can clearly see or feel that they're not the same. If there's an appreciable amount of something, there's enough of it to actually count for something.

When a teacher says your math skills have shown appreciable improvement, she means you've genuinely gotten better in a way anyone could see. The progress is clear and obvious. If a scientist detects an appreciable change in temperature during an experiment, the thermometer moved enough to matter for the results.

The word often appears when someone wants to emphasize that something is significant and substantial. An appreciable number of students means a significant group, not just one or two. An appreciable amount of time means enough time to actually accomplish something, not just a few seconds.

The opposite would be negligible, meaning too small to notice or matter. If your fundraiser earned an appreciable sum of money, you raised enough to make a real difference for your cause.