A stratagem is a scheme or a clever plot. You can have a stratagem for winning a chess game, getting the girl (or boy), and avoiding a punishment. However, your opponents, crushes, and parents may have a trick or two of their own.
Sometimes a stratagem is a gem of an idea, really clever and worth trying. Great generals start a battle plan with a stratagem, and businesses might have a stratagem for making more money. A stratagem is often a trick or a way to deceive an enemy or get something through a plot or ploy, but it can also mean just a great idea that outwits someone. Your teacher probably has a stratagem for helping you remember and spell words.