A locus is the specific physical location of a gene or other DNA sequence on a chromosome, like a genetic street address. The plural of locus is "loci".
"Locus" is a term that we use to tell us where on a chromosome a specific gene is. So it's really the physical location of a gene or of a DNA polymorphism on a chromosome. And it's sort of like a street address for people. And one of the things that we think about when we're thinking about genes and chromosomes is we may think of the chromosome as a country, and then a region of a chromosome would maybe be the city, and then we'll get down to a very specific area, which is the locus, and that would be equivalent to, say, a person's street address. And that's the street address of that gene. An important thing to remember is that the plural of "locus" is "loci", not "locuses".