n.

1. 牢固；（颜色）不褪色[U]
   We guarantee the fastness of these dyes.

   我们担保这些颜料不褪色。

2. 据点；堡垒[C]
   The rebels have withdrawn to their mountain fastness for the winter.

   叛军撤至他们的山寨过冬。

n.

1. a secure place well protected by natural features.
2. the ability of a dye to maintain its colour without fading or washing away.