1An interjection expressing disbelief or strong denial — 'No way!' or 'You don't say!'
まさか！彼がそんなことをするはずがない。
No way! He would never do such a thing.
まさか、冗談でしょう？
No way — that's a joke, right?
2Adverb used (often with a following negative) to emphasize that something is certainly not the case ('surely not', 'by no means').
まさか彼が犯人なわけがない。
There's no way he's the culprit.
3A noun meaning an unexpected event or emergency (used in expressions like 'in case of the unexpected').
まさかの事故で電車が止まり、帰宅が遅くなった。
Due to an unexpected accident the train stopped and I got home late.
4As an adverb meaning indeed or unexpectedly — used to express surprise that something actually happened.
まさか彼が本当に来るとは思わなかった。
I didn't think he would actually come.
5An archaic/rare usage meaning 'the present moment' or 'the situation at hand' (rarely used in modern Japanese).
古文には『まさかにして』という表現が見られることがある。
In classical texts you can sometimes find the expression 'masaka ni shite'.