glioblastoma multiforme (gl-blas-tm) A glioma consisting chiefly of undifferentiated anaplastic cells of astrocytic origin that show marked nuclear pleomorphism, necrosis, and vascular endothelial proliferation; frequently, tumor cells are arranged radially about an irregular focus of necrosis; these neoplasms grow rapidly, invade extensively, and occur most frequently in the cerebrum of adults. [G. glia, glue, + blastos, germ, + -oma, tumor]