palette knife

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A knife with a thin flexible blade, used by artists for mixing, scraping, or applying paint.

from The Century Dictionary.

• noun A thin, flexible, round-pointed blade set in a handle, used by painters for mixing colors on a palette or on a grinding-slab, and by druggists for mixing salves. These knives are of various forms, according to the uses to which they are put.
• noun In printing, a thin blade of flexible steel, about one inch in width, and six or more inches in length, fitted to a handle, used by pressmen to aid the distribution of printing-ink on any flat surface.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun In painting, a small, edge-less, more or less flexible steel blade used to mix paint on a palette and sometimes to apply paint to a surface.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a spatula used by artists for mixing or applying or scraping off oil paints