From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): ( a.) Having the constituent parts so compact, or so firmly adhering, as to resist the impression or penetration of other bodies; having a fixed form; hard; firm; compact; - opposed to fluid and liquid or to plastic, like clay, or to incompact, like sand.

(2): ( a.) Firm; compact; strong; stable; unyielding; as, a solid pier; a solid pile; a solid wall.

(3): ( a.) Applied to a compound word whose parts are closely united and form an unbroken word; - opposed to hyphened.

(4): ( a.) Fig.: Worthy of credit, trust, or esteem; substantial, as opposed to frivolous or fallacious; weighty; firm; strong; valid; just; genuine.

(5): ( a.) Sound; not weakly; as, a solid constitution of body.

(6): ( a.) Of a fleshy, uniform, undivided substance, as a bulb or root; not spongy or hollow within, as a stem.

(7): ( a.) Impenetrable; resisting or excluding any other material particle or atom from any given portion of space; - applied to the supposed ultimate particles of matter.

(8): ( a.) Not having the lines separated by leads; not open.

(9): ( a.) United; without division; unanimous; as, the delegation is solid for a candidate.

(10): ( n.) A substance that is held in a fixed form by cohesion among its particles; a substance not fluid.

(11): ( n.) A magnitude which has length, breadth, and thickness; a part of space bounded on all sides.

(12): ( a.) Not hollow; full of matter; as, a solid globe or cone, as distinguished from a hollow one; not spongy; dense; hence, sometimes, heavy.

(13): ( a.) Having all the geometrical dimensions; cubic; as, a solid foot contains 1,728 solid inches.

Vine's Expository Dictionary of NT Words [2]

1: Στερεός (Strong'S #4731 — Adjective — stereos — ster-eh-os' )

for which see Firm , No. 2, has the meaning "solid" in  Hebrews 5:12,14 , of food (AV, "strong"). As "solid" food requires more powerful digestive organs than are possessed by a babe, so a fuller knowledge of Christ (especially here with reference to His Melchizedek priesthood) required that exercise of spiritual intelligence which is derived from the practical appropriation of what had already been received.