what is lean ?

Answer 1

Lean can have many different meanings depending of the area you are speaking about. In automotive terms it is a condition where the air/fuel mixture is less than Stoichiometric.
Lean can also be used in manufacturing. Lean manufacturing is a management philosophy focusing on reduction of the seven wastes to improve overall customer value:
Transportation
Inventory
Motion
Waiting time
Over-production
Processing Itself
Defective Product (Scrap in manufactured products or any type of business.)
There is also an illegal drug in the southern US made from cough syrup that is called "Lean". Purple Drank is an illegal recreational drink popular in the Southern United States rap community, whose main ingredients are prescription strength cough syrup containing codeine and promethazine

I hope that this helps.

Answer 2

It may well have other meanings but in absorption systems the adjective 'lean' is applied to the liquid (for example 'lean amine') prior to absorption of other components.