Solitaire is a type of card game that is only played by one player, and there are many different solitaire games in existance. They can use either a single deck or multiple. Some of them also require the jokers, which ups the deck to 54. They can be based around luck of the draw, skill, or a combination of the two. Some are rather quick, or played as a time waster. The card layout, called the tableau, can take up lots of space, or only a little.
On most computers and certain video games, the term "solitaire" refers to one of the most common version, Klondike.