A vaginal probe is a small ultrasound transducer (e.g. a biplane probe) that is inserted directly into the vagina. A sterile cover is slipped over the handheld transducer (also handle or probe), which is then covered with lubricating gel and placed in the vagina.
This technique produces a sharp image, not only because of the close proximity to the uterus, but also because this transducer is more sensitive.