During my PhD, my advisor and I disagreed about the spelling of runtime/run time. There was no definite answer as different papers used different spelling in different usages. This week I found a rationale that makes sense to me, even if it means that we were both wrong.
There are three variants of the word “run time” in computer science:
run-time — adjective (“the run-time performance”)
run time — noun — a moment in time (“occurs at run time”), or an amount of time (“the run time is 8 hours”)
runtime — noun — a program that runs/supports another program (“the runtime handles memory allocation”); a synonym for “runtime system”