Hyperglide is Shimano's terminology for cogs/sprockets with select teeth having a special profile consisting of ramps and a thinner section to aid in the release of the chain when shifting.The result was quicker and easier shifting, especiallly under heavy loads. Though it doesn't sound nice, Hyperglide will even shift to a lower gear at low rpm, when going uphill. It was a major shifting advance.

Since Hyperglide pertains to the cogs, they can be manufactured in either cassette or freewheel designs. Hyperglide freewheels are still being marketed, though only in 6 and 7 speed version with limited cog selection.

Edit: Hyperglide freewheels typically use the number format MF-HGxx, where MF indicates Multiple Freewheel, HG indicates Hyperglide and xx is number designation indcating the level in the Shimano hierarchy.