A sejm court was a court that sat in cases of impeachment – in the words of the May 3 Constitution of 1791 (article VIII: the judicial authority) – of government "ministers [...] charged with breach of law by a deputation designated to examine their deeds [...]."
The composition and functioning of sejm courts were spelled out in an act of the Sejm passed on May 13, 1791.
References
Sejm court Wikipedia(Text) CC BY-SA