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Webster 1913 Edition

Noun.

The act of darkening, or state of being dark; darkness; obscurity.

Johnson.

Noun.

The act of darkening or blackening.

infuscation ‎(countable and uncountable, plural infuscations)

The act of darkening.
The state of being dark; obscurity.
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A dark patch or marking.
- 1959, Kansas Agricultural Experiment Station, Technical Bulletin
  We consider, until more specimens are discovered, interruptus as only a subspecies of poecilogaster as the major differences between the two are color intensity of the wing infuscations, and of pile and tomentum.