Is workout one word or two words?

Answer 1

As a noun, it's one word.

As a verb, it's two: work out.

E.g. "I did a two-hour workout yesterday. I went to the gym to work out."

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The same rule is true for any common combination of a verb and preposition; the noun or adjective is one word, while the verb is two words: "This software is easy to set up because its setup manual is written clearly."

The reason for this rule is that, at least in English, you couldn't form the past tense (-ed) or gerund (-ing) if the verb form were a single word; you'd end up with strange combinations like "workoutted" or "setupping"!