A locus is a location or position where something happens or exists. Scientists might describe the locus of an earthquake as the exact spot underground where the shaking started. A detective investigating a crime identifies the locus of the incident to understand what happened.

You'll encounter it most often in mathematics and science. In geometry, a locus describes all the points that satisfy a particular condition. For example, a circle is the locus of all points that are exactly the same distance from a center point.

In biology, a locus refers to the specific position of a gene on a chromosome, like an address telling you exactly where to find something in a vast library. Medical researchers studying disease might identify the locus of a genetic mutation.

The plural is loci (pronounced LOH-sigh). When you see locus in your reading, it signals that the writer is being very specific about location, not just gesturing vaguely at an area. Think of it as the difference between saying “somewhere in the neighborhood” versus giving an exact street address.