mrawesome22 states:
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The distance between the case head and bolt face is the headspace.
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While that's a popular belief, the firearms manufacturing, gunsmithing and reloading tool industry standard definition for that distance (or dimension) is "head clearance." But I well understand why folks use "headspace" to refer to that space between the bolt face and case head. It's just that dimensional specifications for headspace gages state the distance between the bolt face and the reference point in the chamber that stops the case from going too far into the chamber.
Head clearance for a given chamber is determined by subtracting case headspace, the distance between the case head and the industry standard point on the case used as a reference (shoulder on rimless bottleneck cases, front of the rim on rimmed cases, front of the belt on belted cases and case mouth on rimless straight cases), from chamber headspace.
One example, a .308 Winchester case with 1.627" case headspace (head to shoulder reference) in a SAAMI spec minimum chamber with a chamber headspace of 1.630" will have a head clearance of .003" when the round fires.
Another example, a .300 Win. Mag. case with .217" case headspace (head to front of belt) in a SAAMI spec minimum chamber with a chamber headspace of .220" will have a head clearance of .003" when fired.
A historical example, a .30-30 Win. case with .060" case headspace (head to front of rim) in a SAAMI spec minimum chamber with a chamber headspace of .063" will have a head clearance of .003" when fired.
A military example, a .30 Carbine case with 1.287" case headspace (head to front of mouth) in a mil spec minimum chamber with a chamber headspace of 1.290" will have a head clearance of .003" when fired.
The above aside, folks choosing to use different meanings for "headspace" may do so, but it makes understanding difficult by others who choose to use the languange of industry standards.