A fundamental ingredient.
A necessary ingredient.
Necessary.
Necessary for survival but not synthesized by the organism, thus needing to be ingested.
Being in the basic form; showing its essence.
Really existing; existent.
Very important; of high importance.
Having the nature of essence; not physical.
Such that each complementary region is irreducible, the boundary of each complementary region is incompressible by disks and monogons in the complementary region, and no leaf is a sphere or a torus bounding a solid torus in the manifold.
Idiopathic.
A principal vein into which a drug can be injected.
In longline fishing the central line to which the branch lines with baits are attached.
An airline's main operating unit, as opposed to codeshares or regional subsidiaries.
The principal route or line of a railway.
The main repository for a software project, from which different versions (forks) may be split off.
The pipeline carrying wastewater to the public drains or a septic tank.
To integrate (code, etc.) into the main repository for a software project, rather than separate forks.
To inject (a drug) directly into a vein.
To consume voraciously.
Normal, principal or standard.
Of or pertaining to the principal route or line of a railway.
Of or pertaining to a surface railway as distinct from an underground, elevated or light rail one.
Of a sequence of opening moves: being part of a main line ("a standard sequence of opening moves considered to be best play").