fastness
noun
[ ˈfɑːs(t)nəs ]
• a secure place well protected by natural features.
• "a remote Himalayan mountain fastness"
• the ability of a material or dye to maintain its colour without fading or washing away.
• "the dyes differ in their fastness to light"
Origin:
Old English fæstnes (see fast1, -ness).
