The action of sustain, v. (in various senses). Also: an instance or the result of this.

The action of physically holding someone or something up. Also: an object which provides physical support. Cf. sustentation, n. 9a. Obsolete.

Originally: †the quality of being able physically to hold up or keep from falling a person or object (obsolete). Subsequently: the action of…

? Support.

Physical or material holding or propping up; = support, n. 4. Also in figurative context. Obsolete.

The action or fact of holding up, keeping from falling or giving way, or bearing the weight of something; the condition of being so supported. Cf…

The action or fact of supporting a person or thing, or of being supported (in various senses of support, v.); support.

A centre of gravity; a fulcrum; a prop, a support (literal and figurative).

(under-, prefix1 affix 2b(c).)