Having only one of the pair of genes, DNA segments or whole chromosomes from a pair of homologous chromosomes.

having only one chromosome of a homologous chromosome

Refers to a cell or individual that is basically diploid but that has only one copy of one particular chromosome type and thus has chromosome number 2

diploid cell missing a single chromosome. A cell or individual that is basically diploid but that has only one copy of one particular chromosome type and thus has chromosome number 2n - 1.