meaning of TDC and BDC in four stroke engine

what is TDC and bdc in four stroke engine

TDC means top dead centre and bdc means but I'm dead centre in four stroke engine Pistons moves in cylinder from upper and lower end the upper end of cylinder that is where the spark plug and inlet outlet was amount it is called top dead centre and the lowest part of the cylinder is called a bottom did of the cylinder Piston moves in the cylinder in between these two ends in four stroke engine 8 stroke the Piston moves although from TDC to bdc or bdc to PDC in example in power stroke Piston moves from TDC to bdc