Completely means totally, entirely, with nothing left out or missing. When you completely finish your homework, you've done every single problem. When a glass is completely empty, there's not a drop left inside. When you completely disagree with someone, you don't agree with them even a little bit.

The word emphasizes thoroughness. If your room is completely clean, you've picked up, organized, and put away everything properly. If you completely understand a math concept, you grasp it from top to bottom, including all the details and how they connect.

Completely intensifies whatever it describes. A cake can be delicious, but a completely delicious cake is absolutely, perfectly delicious in every bite. A story can be ridiculous, but when it's completely ridiculous, it's absurd beyond belief.

The opposite would be partially, somewhat, or incompletely. You might partially finish a puzzle, leaving a few pieces undone, but when you completely finish it, every piece is in place. The word signals that something is done all the way, with no gaps, no exceptions, and nothing missing.