Solitaire refers to a family of single-player card games of a generally similar character, but varying greatly in detail. (The term "Patience" is more commonly used in British English; while Solitaire means peg solitaire.)
These games typically involve dealing cards from a shuffled deck into a prescribed arrangement on a tabletop, from which the player attempts to reorder the deck by suit and rank through a series of moves transferring cards from one place to another under prescribed restrictions.
Solitaire has its own terminology; see solitaire terminology.
There are many different solitaire games, but the term "Solitaire" is often used to refer specifically to the most well-known form of the game, properly called "Klondike". Klondike and some other solitaire games have been adapted into two-player competitive games.
There are a vast array of variations on the solitaire/patience theme, using either one or two decks of cards, and with rules of varying complexity and skill levels. Many of these games have been converted to electronic form and are available as computer games. A basic form of Klondike solitaire comes with every installation of Microsoft Windows, for example. A two-deck electronic example is Arachnid Solitaire.
The term 'solitaire' is also used for single-player games of concentration and skill using a set layout of tiles, pegs or stones rather than cards. These games include: