The number of full chromosome sets in a cell is indicated as a multiple of n. A eukaryotic cell with two complete sets of chromosomes is therefore described as being "2n" (or "diploid").
After DNA replication in the S phase, the cell's DNA content doubles, and it now contains four complete copies of the genome, making it "4n." Then, after the cell divides in the M phase, there are two daughter cells, each with two complete sets of chromosomes (2 x 2n cells).
Most cells cycle between being 2n and 4n. Sperm and egg cells (gametes), however, are special cases: They are "haploid" and contain only one set of chromosomes, making them 1n or simply n. Gametes therefore cycle between being 1n and 2n.