Instantly means immediately — at once, with no delay. It is the ordinary adverb from instant, and students meet it early: come instantly, I knew instantly, the pain stopped instantly. It is at home in speech and in writing. Though actually, people stretch it. ‘I’ll be there instantly’ often means ‘very soon’, not ‘in zero seconds’, the same way 馬上 in Taiwanese talk is not always a stopwatch. Older English also used instantly as a conjunction meaning ‘as soon as’ (‘instantly he arrived, the noise stopped’), but that use is now rare and bookish; you can leave it on the shelf. Compared with instantaneously, this is the one you actually say. Very old texts can still use instantly to mean ‘urgently’ (‘I pray you instantly’), but modern English has dropped that.