Definition: Def. 11.05: Inclination of a Straight Line to a Plane
The inclination of a straight line to a plane is the angle contained by the drawn and standing (straight lines), when a perpendicular is lead to the plane from the end of the (standing) straight line raised (out of the plane), and a straight line is (then) joined from the point (so) generated to the end of the (standing) straight line (lying) in the plane.
Modern Formulation
(not yet contributed)
Thank you to the contributors under CC BYSA 4.0!
 Github:

 nonGithub:
 @Fitzpatrick
References
Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"
Adapted from CC BYSA 3.0 Sources:
 Prime.mover and others: "Pr∞fWiki", https://proofwiki.org/wiki/Main_Page, 2016