What defines the trigone of the urinary bladder?

Trigone of the urinary bladder is made from three orifices, two ureteric orifice and one urethra orifice.

Ureteric orifice is made by ureter from the left and right kidney. It's the place where urine enters the bladder.

Urethra orifice is connected to urethra and it's where the urine comes out of the bladder.