The HGP aimed to map a reference genome with one version of each chromosome, and that ploidy term is haploid.
Human body cells normally contain two chromosome sets—one inherited from each parent—so they are diploid, with 46 chromosomes. A haploid set contains 23 chromosomes: one copy of each autosome and one sex chromosome. Sperm and eggs are haploid.
The human reference genome is a standardized sequence, not the complete genome of one particular person. It is also not a typical diploid genome containing both parental copies. Researchers use it as a coordinate system for locating genes, comparing DNA sequences, and identifying variants.
“Haploid” is sometimes confused with “single-copy” in a broader sense. Here, it specifically describes the chromosome-set representation. The original project produced a composite reference assembled from samples from several people, and later reference assemblies have corrected errors and filled gaps.