Never mind, as two words, is a conjunction. It is usually used between two options, ideas, or situations, with the latter being the less probable. It is synonymous with much less or let alone.
Another definition for never mind is when the words are used almost as an interjection to tell the listener not to worry. Most commonly it is used when the audience is trying to figure out a concept or idea, or even simply what the speaker said. It is almost synonymous with forget about it or it doesn’t matter. Some dictionaries list this definition under never mind, while others don’t list it at all. On the other hand, most speakers have gone the way of simplification and merged the two-word conjunction into nevermind.
Spell checkers will flag nevermind, but it is used once for every two times never mind is used, which means it’s pretty popular. It is only a matter of time before it is added to the dictionary as an official listing. Meanwhile, in informal language it is fine, but stick to never mind for formal work. Though the odds of telling your audience in a formal work to forget about it, is unlikely. Perhaps that is why the written definition is lacking.