When the entire snowpack slowly moves as a unit on the ground, similar to a glacier.
Glide avalanches:

Glide occurs when the entire snowpack slowly slides as a unit on the ground, similar to a glacier. Don't mistake glide for the catastrophic release of a slab avalanche that breaks to the ground. Glide is a slow process, that usually occurs over several days. Glide occurs because melt water lubricates the ground and allows the overlying snowpack to slowly "glide" downhill. Usually, they don't ever produce an avalanche but occasionally they release catastrophically as a glide avalanche. So the presence of glide cracks in the snow do not necessarily mean danger. It's often difficult for a person to trigger a glide avalanche but at the same time it's not smart to be mucking around on top of them and especially not smart to camp under them.

We tend to find them in wet climates and when they occur in dry climates they do so in spring when water percolated through the snow or sometimes during mid winter thaws.

When do they come down? Like an icefall, they come down randomly in time--when they're good and ready--not before. You would think that they would come down during the heat of the day or when melt water running along the ground reaches its maximum. But oddly enough, they tend to release just as often with the arrival of cold temperatures following melting as during melting itself. It's hard to play a trend with glide avalanches. They come down when they're good and ready and it's impossible to tell when that is. Just don't spend much time underneath them.

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab