(surgery) An opening that occurs naturally or is created surgically, as through a biological membrane.
(botany) Either a translucent area in a structure such as a leaf or flower petal, or an opening in a structure.
2005 S. Tucker, Australian Systematic Botany 19(3):193–210. p. 194
The terms ‘nectar window’ and ‘fenestration’ are used interchangeably for enlarged gaps on the adaxial side of the stamen sheath or tube. The fenestrations facilitate access to the nectaries by a pollinator.
2016 D. W. Armitage, Systematic Botany 103(4):780–785. p. 781
Darlingtonia leaves have fenestrations.
Foliar traits unique to Darlingtonia include the presence of transparent light-transmitting fenestrations, ...
(nautical) The practice of placing holes in the rudder of a ship to reduce the work required to move the rudder while preserving its ability to steer the ship.