Increase or decrease in loci found within an organism's genome.
Number of loci can increase via gene duplication or instead as a consequence of acquisition of new loci via horizontal gene transfer.
Number of loci can decrease due to large deletion mutations. The number of functional loci can also decrease due to the fixation of gene-inactivating mutations, resulting particularly in the formation of what are known as pseudogenes.
Most genomes can be viewed as existing at a steady state where new genes/loci come into being at approximately the same rate as genes/loci are lost from the same genomes. If this balance is not present, then the gene complement associated with a given organism will be on the increase or, instead, may be decreasing. Depending upon the organism, increases in gene number may be more a function of gene duplication then horizontal gene transfer, or vice versa. See also the concept of new alleles.
Depending on the gene as well as circumstances, loss or inactivation of loci may be resisted by natural selection, meaning that those individuals within which these mutations occur will be at a selective disadvantage to those in which loci instead remain intact. Alternatively, inactivated loci may become fixed as a consequence of genetic drift even if the underlying mutations are detrimental to the carriers.
Note that gene acquisition too can be detrimental to carriers, that is, gains in numbers of loci (versus losses). In many cases, though, such gains are likely instead to be neutral, that is, rather than beneficial or instead detrimental.