(1) To prove; to try a substance; to determine the chemical nature of a substance by means of reagents. (2) A method of examination, as to determine the presence or absence of a definite disease or of some substance in any of the fluids, tissues, or excretions of the body, or to determine the presence or degree of a psychological or behavioral trait. (3) A reagent used in making a test (4)testa (1)assay, reaction, reagent, scale, stain. [L. testum, an earthen vessel]