What is the term for a change in the relative frequencies of alleles in a population over several generations?

Microevolution

Explanation:

Change in allele frequencies through time occurs via evolutionary mechanisms: mutation, natural (and artificial) selection, genetic drift, gene flow (migration).

Macroevolution also represents the change in allele frequencies but on larger time scale: while microevolution is at species level, macroevolution  refer to all levels above species.