That’s an American English thing. In most of Europe and British English coriander refers to the plant/leaves, and the seeds are just called coriander seeds. Etymology from wiki:

First attested in English during the late 14th century, the word “coriander” derives from the Old French coriandre, which comes from Latin coriandrum,[15] in turn from Ancient Greek κορίαννον : koríannon (or κορίανδρον : koríandron),[16][17] possibly derived from or related to κόρις : kóris (a bed bug),[18][19] and was given on account of its fetid, bug-like smell.[20]

The earliest attested form of the word is the Mycenaean Greek ko-ri-ja-da-na[21] (variants: ko-ri-a2-da-na, ko-ri-ja-do-no, ko-ri-jo-da-na)[22] written in Linear B syllabic script (reconstructed as koriadnon, similar to the name of Minos’ daughter Ariadne) which later evolved to koriannon or koriandron,[23] and Koriander (German).[24]

Cilantro is the Spanish word for coriander, also deriving from coriandrum. It is the common term in US English for coriander leaves due to their extensive use in Mexican cuisine, but the seeds are referred to as coriander in American English.[24]