Ornamental means put there for decoration rather than for use. An ornamental pond, ornamental grasses, purely ornamental.
Calling somebody's role ornamental is a real insult, since it says they have no function. In gardening it is neutral and simply distinguishes plants grown for looks from those grown to eat.
1adjectiveornamentalPut there for decoration rather than for use.
“The garden has an ornamental pond with a small stone fountain.”
ornamental is a quality.
Proto-Indo-European
*h₂er-
Latin
ornamentum — equipment, apparatus, furniture, trappings, adornment, embellishment
Old French
ornement
Middle English
ornament
English
ornament
English, today
ornamental
Built from
From ornament + -al. ornament comes from Middle English ornament, from Old French ornement, from Latin ornamentum (“equipment, apparatus, furniture, trappings, adornment, embellishment”), from ornō (“to equip, adorn”). The verb is derived from the noun.
Etymology from Wiktionary, CC BY-SA