What is the doctrine of secondary meaning?

This doctrine is to the effect that a word or phrase originally incapable of exclusive appropriation with reference to an article on the market, because it is geographical or otherwise descriptive, may nevertheless be used exclusively by one producer with reference to his article so long as in that trade and to that branch of the purchasing public, the word or phrase has come to mean that the article was his product. (G. and C. Merriam Co. v. Saalfield, 198 F. 369, 373, cited in Ang v. Teodoro, G.R. No. L-48226, Dec. 14, 1942)