is a specific kind of: quality.

is a broader term that covers: effortfulness | asperity, grimness, hardship, rigor, rigorousness, rigour, rigourousness, severeness, severity | hardness, ruggedness | formidability, toughness | burdensomeness, heaviness, onerousness, oppressiveness | niceness, subtlety | inconvenience, troublesomeness, worriment.

is an attribute of: difficult, hard.

is the opposite of: ease, easiness, simpleness, simplicity.