Other forms: solids
Solid means hard and firm all the way through — not liquid, not gas, and not hollow. A solid wall, solid oak, water freezes into a solid.
It has a second everyday sense meaning reliable or substantial: a solid performance, solid evidence, a solid three hours of work. That use is a compliment and it is very common. Note solid gold, which means gold all the way through rather than plated, and the phrase solid ground, both literal and figurative. In geometry a solid is a three-dimensional shape, and in chemistry it is one of the three states of matter — solid, liquid, gas — which is where school introduces the word.
1adjectivesolidHard and firm, not liquid or gas.
“The pond was frozen solid.”
2adjectivehollowNot hollow, and made of the same material throughout.
“The table is solid oak, not veneer.”
3adjectivereliableReliable and substantial.
“It was a solid piece of work and nobody could fault it.”
4nounsubstanceA substance in the state that is neither liquid nor gas.
“At that temperature it is a solid.”
Too few real examples of this word to estimate which sense is commonest.
solid is a state of matter, or a quality of firmness.
Latin solidus meant whole and entire, from a root meaning complete — the same root as safe and salute, both of which are about being whole. A Roman solidus was a gold coin, and the word survives in the old British abbreviation s for shilling.
Proto-Indo-European
*solh₂-
Latin
solidus
Old French
solide
Middle English
solide
English, today
solid
Built from
From Middle English solide, from Old French solide, from Latin solidus. From Middle English solide, borrowed from Old French solide, from Latin solidus (“solid”), from Proto-Indo-European solh₂-i-dʰ-o-s (“entire”), suffixed form of root solh₂- (“integrate, whole”). a doublet of sol and sold and soldo and solidus and sou and xu.
Etymology from Wiktionary, CC BY-SA