In spoken English, breakdown and break down are indistinguishable except for the lack of stress on the second syllable of breakdown. They are otherwise pronounced identically. Breakdown is a noun. Break down is a verb. The terms are not interchangeable.

The words breakdown and break dow n both refer to a failure or a dismantling of some kind, whether it's mechanical, physical, or emotional. The difference is that, written as one word, breakdown is a noun, referring to the result of the action, while the two-word version, break down, is a phrasal verb that denotes the action leading to the result.