1. marks or lines left by the highest or lowest level of a tide
Usage: nautical; coastal geography
2. dirty marks or stains left around the edge of something, especially a bathtub
Usage: informal; British

Examples

• The storm left clear tidemarks on the rocks along the shore.
• Scientists studied the tidemarks to understand sea level changes.
• High tidemarks showed where the water had reached during the flood.
• She scrubbed the tidemarks from around the bathtub.
• The old tidemarks on the pier indicated previous high tides.
• Dark tidemarks stained the white porcelain sink.