Used in the context of general equities. For convertibles, trade without accrued interest , preferred stock always "trades flat" as do bonds on which interest is in default or is in doubt. For general, trade in and out of a position at the same price, neither making a profit nor taking a loss.
TRADE FLAT
Meaning of TRADE FLAT in English
Campbell R. Harvey. Hypertextual finance English glossary. Английский словарь гипертекстовых финансов. 2012