Question

What is polyploidy? How are polyploid plants different from diploid plants?

Answer 1

This is yet another case of Latin/Greek roots helping us.
Poly - means many,
Di- means two
Ha- comes from greek (i think) meaning solo or one.

Thus, Haploid = 1 "ploid"; Diploid = 2 and Polyploidy means more than 2
ploid comes from Ploidy which is the number of chromosomes sets within an organism. thus:

Polyploidic plants have at least one more set of chromosomes than diplodic; however, they can contain any number more because we are not saying triploidic.