The Hull is an old term, referring to the body or frame of a starship. Warp-worthy vessels must have a hull able to withstand extreme accelerations. The hull of a Starfleet vessel is usually separated into two parts: The primary and secondary hulls.
Additional hull protection Edit
It must be noted that despite the outer hull of ships being extremely strong, they alone will often not protect a ship, especcially when at warp. Additional protection of the hull can consist of deflector shields, hull armor and a deflector array. Without a deflector to help maintain hull integrity and ward off minuscule particles and atmospheric disturbances a Starfleet vessels hull is very vulnerable and can be breached. (JB: "Just Between Stars")