Hyphenate adjectives, not nouns
"Object-oriented" is hyphenated, but "use case" is not.
- run time – noun, the time at which the system runs (not hyphenated)
- run-time – an adjective describing something which exists at run time (hyphenated)
- runtime – noun, the run-time support, e.g. the VM (portmanteau word combining "run" and "time")
”Run-time errors are caught by the runtime at run time.”
See also: Hyphens Between Words