A market-wide directional movement, or a market condition in which most stocks and sectors are moving in the same direction. These movements are usually caused by market-wide events.

If you hear in the financial media that the "stock market is up across the board", it means that most of the stocks in the market are up on that day's trading. The term comes from the NYSE big board, a large board on which stock prices were once written; when the majority of prices were up or down, the movement was shown "across the board".