1. an earth color containing ferric oxides; used as a pigment;
The Collaborative International Dictionary of English v.0.48:
Sienna \Si*en"na\, n. [It. terra di Siena, fr. Siena in Italy.]
Clay that is colored red or brown by the oxides of iron or
manganese, and used as a pigment. It is used either in the
raw state or burnt.
Burnt sienna, sienna made of a much redder color by the
action of fire.
Raw sienna, sienna in its natural state, of a transparent
yellowish brown color.
WordNet (r) 3.0 (2006):
n 1: an earth color containing ferric oxides; used as a pigment