Lift is a term that is used very frequently here, but I cannot find any post explaining what it is exactly. I am looking for the exact definition of lift, not an explanation of how it is generated.
It seems a very simple question, but I have not been able to find consistent answers to it. Some very well respected sources even contradict each other:
At https://en.wikipedia.org/wiki/Lift_(force) Lift is defined as:
A fluid flowing past the surface of a body exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction.
But according to NASA (https://www.grc.nasa.gov/www/k-12/airplane/lift1.html):
Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air.
If a plane is flying straight and level (at constant altitude) these definitions are the same, but otherwise they are not the same at all. So, which one is correct?