What does "runtime" mean?

This morning I was going through the Elm docs and I came across this line that caused me to pause:

In Elm, commands (Cmd) are how we tell the runtime to execute things that involve side effects.

I understood "runtime" to be the length of time a program took to run, so this sentence confused me. After talking about it with some people from RC, we learned that, depending on the context, "runtime" can mean different things:

Thanks to Juliano Solanho and Chris Ball for their help!

27 Sep 2016