what is memory?

In psychology, memory is an organism's ability to store, retain, and subsequently retrieve information. Memory comes within the paradigms of cognitive psychology. In recent decades, it has become one of the principal pillars of a branch of science called cognitive neuroscience.

So, here memory is nothing but the storage of the input data, at the time of need of the data/solution from the stored data, if you can recall the particular content instantly and effortlessly that means you have a great memory.