hard to understand; recondite; esoteric

abstruse theories.

Obsolete . secret; hidden.

adjective

clear , concrete , easy , lucid , obvious , plain , simple

adjective

abstract , clear as dishwater , complex , complicated , deep , enigmatic , esoteric , greek to me , heavy * , hidden , incomprehensible , intricate , involved , muddy , obscure , perplexing , profound , puzzling , recondite , subtle , unfathomable , vague , arcane , cabalistic , cryptical , dark , difficult , hard , ideal , inaccessible , inscrutable , metaphysical , mysterious , mystic , nebulous , remote , secret , stratospheric , tangled , unknown