A standardized number of shares defined by a stock exchange as a trading unit. In most cases, this means 100 shares. The purpose of a board lot is to avoid "odd lots" and to facilitate easier trading. It's more difficult for a broker to find a buyer for, say, 17 shares, than if everybody agrees to trade in 100 share lots.




Taobiz explains Board Lot
For example, a stock exchange might define one board lot as equaling 1,000 shares for stock priced under $1, and 100 shares for shares of more than $1. The thinking is that standardization increases liquidity thus lowering spreads and making the market more efficient for everybody.