What term describes an organism or cell that has only one complete set of chromosomes?

The term that describes an organism or cell with only one complete set of chromosomes is "haploid." In a haploid condition, an organism or cell contains half the number of chromosomes typical for that species, which is critical during processes like meiosis.

For example, in humans, the diploid number is 46 chromosomes, meaning that cells in somatic tissue contain pairs of each chromosome. In contrast, gametes (sperm and egg cells) are haploid, containing only 23 unpaired chromosomes. This reduction in chromosome number ensures that when gametes fuse during fertilization, the resulting zygote restores the diploid number of chromosomes.

The other terms refer to different genetic concepts. Diploid refers to cells that have two complete sets of chromosomes. Heterozygous describes an organism with two different alleles for a specific gene, while homozygous refers to having two identical alleles for a gene. These concepts are important in understanding genetics, but they do not specifically relate to the defining characteristic of haploidy.