Linked from
The 8 pages that link to Ploidy, each with the reason it gives.
MeiosisNarrower topic: Meiosis changes ploidy from diploid to haploid.
PolyploidyNarrower topic: Polyploidy is a high-ploidy state, defined relative to diploidy.
HaploidNarrower topic: Haploid names the ploidy level with one chromosome set.
DiploidNarrower topic: Diploid is the ploidy state with two complete sets.