Strictly speaking, a hardgainer, as I define it is simply someone who has a difficult time gaining muscle mass. Though this can apply to both ectomorphs and endomorphs, my point of view is that of an ectomorph. Wikipedia defines them as follows:
Ectomorphic: characterized by long and thin muscles/limbs and low fat storage; usually referred to as slim. Ectomorphs are not predisposed to store fat or build muscle.
Endomorphic: characterized by increased fat storage, a wide waist and a large bone structure, usually referred to as fat. Endomorphs are predisposed to storing fat.
These are still ambiguous terms though, so as a way to concretely define hardgainers, I will use the following equations, source here:
wrist <= 0.1045 x height
ankle <= 0.1296 x height
All measurements are in inches. Wrist circumference is measured on the hand side of the styloid process. (The styloid process is the bony lump on the outside of your wrist. Ankle circumference is measured at the thinnest point.
Percentages will vary and should approximate the degree of “hardgainer-ness” that you have. For example, I am 72.5 inches talls with a wrist circumference of 7 inches and an ankle measurement of 9.5 inches. These measurements roughly indicate that i’m a hardgainer in the upper body and an “easygainer” in the lower body.