There are two ways to look at headspace. First, let's consider the practical definition, and follow up with the technical one.
In practical terms, headspace is the clearance allowed between the base of the cartridge case and the face of the bolt. The position of the cartridge in the chamber is controlled in a variety of ways depending on the type of cartridge case involved. Most rimless automatic pistol cartridges (e.g., 9x19mm Parabellum, .45 ACP) are positioned by the case mouth resting on the front edge of the chamber. Rimmed cartridges (e.g., .45 Colt, .30/30 Winchester) are positioned in the chamber by the face of the rim resting on the rim recess at the back of the chamber. Belted magnum cases (e.g., .300 Win Mag., .458 Win Mag.) are positioned by the belt resting on the recess provided for it at the rear of the chamber, much as rimmed cartridges are. Finally, bottle necked rimless cartridges are positioned by the shoulder of the case resting against the shoulder of the chamber.
So when a cartridge is chambered, it is positioned in one of the above ways. Once positioned, there must be a little free space between the cartridge case base and the face of the bolt, to allow for dimensional tolerances in cases. When the cartridge is fired, the case expands in all directions, including toward the bolt face. If there is the intended clearance space, the case head is not stretched excessively during this expansion. If the space is too much, as the case head is pushed backwards towards the bolt face it may stretch enough for it to significantly weaken the case in the area just in front of the thick portion of the case head, called the web, where the thinner walls of the powder containing part of the case begin. If the case head does not separate on the first firing, the weakened brass may do so on subsequent firing. This is a very bad thing, as hot gas at 50,000 psi will damage at least the stock and magazine, if not the firer's hands or face. Little drops of molten brass and brass shards are carried by the hot gas at near-supersonic speeds. If all the firer gets is a Chicken Pox-like tattoo, he or she is fortunate.
Conversely, too little clearance is a bad thing, too. That is, if there is "negative" clearance and the case has to be forced into the chamber by the bolt, it can wedge the case neck tightly around the bullet, raising pressures by thus delaying bullet release. Then the hot gas will come out the primer pocket...
The technical definition of "headspace" for bottle-necked rimless cases is the dimension between the bolt face and the datum line on the chamber or cartridge shoulder, whichever is being referenced. This is the source of the dimensions found in the Bruce Woodford chamber drawing below:
Click on the picture for the full size version.
The "datum line" is a position on the shoulder defined by the military or SAAMI drawing as appropriate. One measures this dimension by means of a chamber headspace gauge. These can be graduated in explicit headspace dimensions (as are Match Gauges for the .308 Winchester) or in qualitative terms: Go, No-Go, and Field. (Chamber headspace gauges are available, of course, from Fulton Armory; see the Parts and Accessories Page for your rifle of interest!) The amount of clearance allowed for a nominally dimensioned cartridge can be inferred by this measurement of headspace. Of course, the actual headspace obtained (i.e., the clearance between the base of a chambered cartridge and the bolt face) depends on the "headspace" dimension of the cartridge, which can be measured by a *cartridge* headspace gauge