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/ˈfɑːs(t)nəs/ · noun
(uncountable) The quality or state of being fast (in various senses)..
Use fastness when its meaning, tone and grammar fit the full sentence. A synonym is not always a direct replacement.
Fastness means (uncountable) The quality or state of being fast (in various senses)..
Common synonyms include stronghold, speed, fixity, swiftness.
The opposite depends on the specific sense.
Use fastness where its definition matches the intended context.
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