Weak layers that continue to produce avalanches for several days or weeks after a storm.
Persistent Weak Layers:

Certain weak layers tend to stabilize quickly after a storm while other kinds of weak layers take much longer to stabilize. The three most notorious, persistent weak layer include: faceted snow, depth hoar and surface hoar. As you can imagine, persistent weak layers cause most avalanche accidents because the avalanche danger can linger several days after a storm, just waiting for a trigger.

The presence of a persistent weak layer, alone, doesn’t necessarily mean danger. But If a buried, persistent weak layer also produces a high quality shear and is also weak on your strength tests, you should definitely avoid avalanche terrain where those conditions exist.

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