Flat is a generic term used in the trading industry to denote several scenarios.  If the price of a share or financial instrument is neither rising nor falling, it is trading flat.  The overall market, such as the Dow Jones Industrial Average, could be up or down a mere couple of points, and people might refer to it as having traded flat on the day.  Occasionally the term refers to a bond that is trading without accrued interest.  Most frequently the term is used to describe an account which doesn't have any positions.  An account with no current positions is considered to be "flat".  For example, if your account had no positions entering Tuesday, and on Tuesday you bought 10 ES futures, and then sold the same ES futures at some point that day, you would not only have traded flat in the ES, but the account as a whole would be flat.