Basic definition: A long-term plan in chess, such as exploitation of weak pawns and weak pawn structures, king-side attack, creation of a pawn majority, creation of knight outpost, Bishop vs Knight Supremacy, trading into a winning endgame, etc. that allows one side to gain an advantage over the other opponent if played correctly.
Defined best by Harry Golombek, chess strategy, or planning, is "the process by which a player utilizes the advantages and minimizes the drawbacks of his position. In order to promise success, planning is thus always based on a diagnosis of the existing characteristics of a position; it is therefore most difficult when the position is evenly balanced, and easiest when there is only one plan to satisfy the demands of the position."
According to International Master Jeremey Silman, the existing characteristics, also called imbalances, preferably assessed by experienced chess players to create an ideal plan are:
1. Superiority of Minor Pieces
2. Pawn Structure
4. Material Advantage
5. Control of a key file or square
6. Lead in development
"Figure out the side of the board you wish to play on. You can only play where a favorable imbalance or the possibility of creating a favorable imbalance exists. Only now do you look at the moves you wish to calculate." --- IM Jeremy Silman.
Dude, listen to this chess strategy I used against Mr. Limbian: I traded my c-pawn for his d-pawn to make his e-pawn an isolated, weak d-pawn, then I piled up my big pieces against that pawn and finally won it. I wanted use the principle of two weaknesses on him, but he barely had any understanding of chess so it didn't matter. The rest of the game was easy; trade all pieces and win the end game. It ended up in a king and pawns vs king and pawns ending, but he understood absolutely nothing about opposition or trianglulation. I basically killed him after that. What an amateur! Want to see the game?