To blur means to make something unclear, smudged, or difficult to see sharply. When you take a photograph while moving, the image comes out blurred. When you rub your eyes after waking up, everything looks blurred until your vision clears. A window covered in condensation blurs the view outside.

The word can also describe when differences or boundaries between things become less clear. When someone works so much that the days blur together, they can't remember which day was which. When a movie has so much action that scenes blur, events happen too fast to follow clearly. Sometimes the lines blur between two ideas, making it hard to tell them apart. For example, as technology advances, the line between computers and phones has blurred: your phone now does almost everything a computer can do.

A blur (as a noun) can be something unclear or a period of time you can't remember well. Someone might say their busy week was just a blur if it passed so quickly they barely remember what happened. It can also mean something moving too fast to see clearly: “The race car was just a blur as it passed by.”