Expanding on this, "solved" specifically refers to a game where the optimal strategy always results in winning or a draw, regardless of what the opponent does, becoming deterministic. It comes from mathematics, where problems are solved, and game theory (the branch of mathematics concerning solutions/proofs dependent on user choices).

Checkers, tic-tac-toe, Mancala, Connect Four, Rubik's cubes, and Othello have all been solved. Go, chess, and Klondike solitaire remain unsolved, despite deterministic board states. It is likely go will never be solved. Solving chess runs into problems with processing and memory, since there's so many possible games. We literally can't mark all of them down if the entire universe inscribed a different game on every atom in existence. With solitaire, we don't even know the probability of winning any given game (much to the frustration of mathematicians).