A blotter is a thick piece of absorbent paper designed to soak up wet ink. Before ballpoint pens became common, people wrote with fountain pens or dipped pens in ink bottles. The ink took time to dry on the page, so writers kept a blotter nearby to press gently against their writing and absorb the excess ink. This prevented smudging and let them turn the page or close their books without ruining their work.

Many desks once had blotting paper built right into them, with special corners to hold it in place. Some people still use blotters today when writing with fountain pens or doing calligraphy.

The word also refers to a large book or ledger where information gets recorded as it happens. Police stations keep a police blotter where officers write down every arrest, complaint, and incident throughout the day, creating a permanent record. You might see excerpts from the police blotter published in local newspapers, listing unusual or interesting calls the police received.