The term pelvis in medicine has several meanings.
The bony pelvis are the 3 bones that form a ring at the bottom of the spine. At the back the spine connects with the sacrum. At each side the two iliac bones join with the sacrum and then with each other at the front at the pubis or pubic bone.
The hip joints are where the thigh bones or femur join with the iliac bones.
The pelvis can also refer to the area of the body inside the bony pelvic ring. Above the pelvis is the abdominal cavity. The pelvis then contains the rectum and bladder, and in the female ovaries and uterus.
The renal pelvis:
The renal pelvis is the name given to the top of the ureter. The ureter is the tube that takes urine from the kidney and takes it down the back of the abdomen, and into the pelvis (pelvic area as above) and then into the bladder. The kidneys are at the back but at the level of the lower ribs. Therefore the pelvic ureter (or ureteric pelvis) is up at this level