A mental technique used as an aid in remembering anything, utilized first by the Greek poet Simonides of Amorgos who used the technique to identify mutilated guests at a dinner party by where they had been sitting before the roof caved in, and most famously in real life by Matteo Ricci and fictitiously by Hannibal Lecter. In this technique a person visualizes a room or a building or an entire city, all of which is filled with objects that lend themselves to mnemonic use, or are associated strongly with a memory. It is said that Ricci could recite a list of four or five hundred Chinese ideograms forwards or backwards after reading or hearing them once.
It is suggested that the objects in a memory palace be imagined in the most meticulous detail in order to sharpen the mind and also to provide room for many things to be accessed by the memory. For example, instead of using a painting of the sea to remember your vacation at the beach, make the sea only a small part of the painting, and also include information about your vacation last year in the Bahamas(perhaps a palm tree by the sea) as well as the time you celebrated the winter holidays in Canada(some snow and pines) and whatever else you can think to put in the painting. Instead of imagining a simple stick drawing of a woman wearing a mortarboard carrying nine pizzas to remember the order of the planets(My Very Educated Mother Just Served Us Nine Pizzas), picture the situation in fine detail, down to the folds of clothing and the toppings on the pizza. The more detailed and extraordinary your imagined objects, the more likely it is that you will remember them.
As Jaez wrote above, to maintain a memory palace one must visit it constantly, but once properly established, it is one of the most effective mental tools.