Fixation is the process by which a new allele replaces the allele that was previously predominant in a population. Defined as key term in Molecular Biology 1.4.4 Sequence divergence is the basis for the evolutionary clock

when the frequency of an allele in a population becomes 1 (i.e., no other alleles are present).

The situation when a single allele reaches a frequency of 100% in a population.

describing an allele for which no alternative alleles at that locus exist in the population. This means that the allele has a frequency of 1 (or 100%) in the population.

loss of all alleles of a gene but one

The point at which a particular allele becomes the only allele at its locus in a population - the frequency of the allele becomes one.

the status of a locus in which all members of a population are homozygous; the frequency of the fixed allele is 1.0

In population genetics, fixation occurs when every individual within a population has the same allele at a particular locus. The allele, such as a single point mutation or whole gene, will be initially rare (e.g. originating in one individual), but can spread through the population by random genetic drift and/or positive selection. Once the frequency of the allele is at 100%, being possessed by each member, it is said to be "fixed" in the population.