What's the difference between a polyploidy and a trisomy?

An organism that is produced by sexual reproduction receives a set of chromosomes from each of its parents' sex cells or gametes. These gametes are known as haploid with a single set of chromosomes. After fertilization occurs, an organism is considered to be diploid which refers to the two sets of chromosomes it received from its mother and father.  A polyploid organism contains additional sets of chromosomes; for instance, if it has three complete sets it is referred to as triploid or it can have four sets which is known as a tetraploid organism. The reason this occurs is due to nondisjunction, a failure of homologous chromosomes to properly separate during gamete formation. This can occur during meioisis I or meioisis II and results in abnormal gametes containing too many or too few chromosomes. If a gamete is diploid rather than haploid and unites with a normal gamete, it can produce a triploid offspring. Or, if two diploid gametes unite, the resulting organism would be tetraploid. Extra sets of chromosomes are generally tolerated more in the plant kingdom than in the animal kingdom and some giant varieties of fruits are actually polyploids. The term trisomy refers to a particular chromosome pair. Rather than the normal two copies of the chromosome in question, one provided by each parent, there are three. Again, a nondisjunction event during meioisis can cause the mother's egg or the father's sperm to have two copies of a particular chromosome. An example of a trisomy in humans is Trisomy 21. On chromosome pair 21, there is an additional chromosome which makes 3 copies of chromosome 21 in the offpsring. The resulting offspring will have a number of characteristic symptoms called a syndrome; in this case, the condition is called Down's Syndrome. There are other trisomies that can occur: for example, there are females with three X chromosomes, there is XXY or Klinefelter syndrome in males who have an additional X chromosome. 
https://en.wikipedia.org/wiki/Polyploid

https://www.differencebetween.com/difference-between-aneuploidy-and-vs-polyploidy/