euploidy
Euploidy means having a chromosome number that is a whole, balanced multiple of the basic set — every chromosome present in the same number of copies, with no odd ones out. If aneuploidy is a card deck missing or carrying a stray card, euploidy is a complete, evenly stacked deck. A normal human diploid cell, with two of each chromosome, is euploid.
The basic set is called the haploid number, written n; in humans n is 23. Euploid states are exact multiples of n: haploid (n) as in a gamete, diploid (2n) as in most body cells, and polyploid states like triploid (3n) or tetraploid (4n) where whole extra sets are present. What matters is that all chromosomes change together, keeping their relative balance.
Because euploidy keeps the dosage of all genes proportional, balanced multiples are far better tolerated in some organisms than the lopsided imbalance of aneuploidy. Many plants are healthily polyploid, and polyploidy has been a major force in plant evolution. In humans, however, whole extra sets such as triploidy are generally not survivable, so for people euploidy effectively means the normal diploid count.
A human gamete with 23 chromosomes (n) and a body cell with 46 (2n) are both euploid; bread wheat, with six full chromosome sets, is a healthy natural hexaploid.
Whole-set multiples keep every gene in proportion — the definition of euploidy.
Euploidy means an exact multiple of the haploid set (n, 2n, 3n ...); aneuploidy means a few chromosomes off from such a multiple. Triploidy is euploid yet lethal in humans, so balanced does not always mean harmless.