Half speed master

Half-speed mastering is done to move the resonant frequency of the cutterhead up one octave. This makes it a lot easier to deal with. Decca developed this technique in the 1950s; many Londons and Deccas are 1/2 speed mastered.

The first answer was the right one:

Half-speed" mastering refers to slowing the cutting lathe to half-speed while cutting the album stamper. This resulted in a more accurate and deeply etched groove that improved bass response and produced more dynamic sonics.

This topic has been discussed at great lengths in Vinylengine.com. According to a Warner Bros mastering engineer who is a member out there, half-speed mastering is merely a marketing gimmick. It sounds nice only because a lot of care has been taken to master the record. Half speed mastering was required to overcome the limitations of record cutting styluses at that time. Post the 70's however, with helium cooling technology (I think), cutting stylus can cut at very high frequencies beyond 20khz.

OK- any cutter head you can think of has a resonance that occurs at a high frequency. My Westerex 3D (which is what RCA used for many of their Living Stereo releases) for example. No amount of messing with the suspension seems to affect it. To deal with this resonance, LP cutter systems use negative feedback. The feedback is controlled by a separate module of the cutting electronics.

However feedback comes with its own problems so it is desirable to use as little as possible (its an adjustment of the feedback module of the cutter electronics). If you run the tape and the lathe at 1/2 speed, you are not going to be putting as much high frequency energy into this resonant peak. This allows you to run less feedback and make a better sounding LP. The fact that Decca was doing this back in the early 60s/late 50s is a testament to how prodigious this issue is.

Later it got turned into marketing hype, and the industry latched on to that, ignoring the actual technical reason it exists, sort of like 'With Retsin' on the side of Certs. Certs does not need to point out that Retsin is vegetable oil. Get my drift? its useful and good, but also over-hyped and thus misunderstood.