What is a partition?

In computer systems, the term "partition" refers to a segmentation or separation of the data storage space on a disk. The most common usage for a hard disk is simply one partition accounting for 100% of the storage space. In a windows system, this would be your drive C:

If, for some reason it was desired to split that drive into two, you could instead make two partitions and split the space 50/50 or 60/40 90/10 whatever, and have drive C: for one and D: for the other. Indeed you can separate the hard disk into dozens of partitions and have drive letters all the way to Z: but this tends to get a bit cumbersome.

Many operating systems prefer to put system files and user files into separate partitions for various reasons, and might also have a 3rd partition for swap files. This is done to boost system performance and stability.

In cases where there are multiple operating systems running on a single computer system, it is common to have each installed on their own partition of the hard disk.