A market-wide directional movement. In other words, 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, so when the majority of prices were up or down, the movement was shown "across the board".
Getting a VIX on Market Direction - Learn about this very useful volatility index of the CBOE, which can tell investors about the mood of the stock market.
Getting to Know Stock Exchanges - Find out the answers to all the questions you had about stock exchanges but were afraid to ask!