primary (adj.)

early 15c., primarie, "earliest, most basic, first in time or sequence;" 1560s, "first or highest in rank or importance," from Latin primarius "of the first rank, chief, principal, excellent," from primus "first" (see prime (adj.)).

The meaning "first or lowest in order of growth or development; elementary, preparatory" is from c. 1800, first in education; primary school is attested by 1793 in translations from French, from école primaire.

The Paris journals ... are full of a plan, brought forward by Fourcroy, for the establishment of primary schools, which is not interesting to an English reader. [London Times, April 27, 1802]

Primary color is attested from 1610s (at first the seven of the spectrum, later the three pigments from which the others can be made). Related: Primarily.

Primary and prime mean first in time, and now especially first in order of importance: as, a primary class, definition, consideration, planet; prime mover, importance, idea .... Primitive means belonging to the beginning or origin, original, hence old-fashioned, having an old-fashioned simplicity: as, a primitive word, the primitive church, primitive purity, manners, unconventionality, dress. ... Primeval means of the flrst or earliest ages, and nothing else. [Century Dictionary] 

primary (n.)

1760, "that which stands first or highest in rank or importance;" see primary (adj.). Meaning "meeting of voters of the same political party in an election district for nominating candidates for office or delegates to a convention" is by 1855, American English, short for primary meeting (1823), primary election (1792, with reference to France; in a U.S. context from 1835), or primary caucus (1817). The system is informal, not constitutional, and has evolved over time.

Theoretically every voter belonging to the party in a district has a right to attend the primary and vote, but in cities and large places only registered voters who have answered certain test questions relating to party adherence have that privilege. [Century Dictionary, 1895]

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Definitions of primary
primary (n.)
a preliminary election where delegates or nominees are chosen;
Synonyms: primary election
primary (n.)
one of the main flight feathers projecting along the outer edge of a bird's wing;
Synonyms: primary feather / primary quill
primary (n.)
(astronomy) a celestial body (especially a star) relative to other objects in orbit around it;
primary (n.)
coil forming the part of an electrical circuit such that changing current in it induces a current in a neighboring circuit;
current through the primary coil induces current in the secondary coil
Synonyms: primary coil / primary winding
primary (adj.)
most important element;
policemen were primary targets
Synonyms: chief / main / principal / master
primary (adj.)
of first rank or importance or value; direct and immediate rather than secondary;
a primary effect
primary sources
primary goals
a primary interest
primary (adj.)
not derived from or reducible to something else; basic;
a primary instinct
primary (adj.)
of or being the essential or basic part;
Synonyms: elementary / elemental / basal