means The set of rules governing how words combine into phrases and sentences in a language (and, in programming, how code must be written to be valid).

from From Greek syntaxis, 'an arranging together,' built from syn- ('together') and taxis ('arrangement, order') — the same taxis behind 'tactics,' where soldiers are arranged in formation. So syntax is literally words drawn up in proper battle order; it entered English in the 17th century by way of Latin and French.