Medical Definition of basal
1: relating to, situated at, or forming the base
2: of, relating to, or essential for maintaining the fundamental vital activities of an organism (as respiration, heartbeat, or excretion) <a basal diet>—see basal metabolism
3: serving as or serving to induce an initial comatose or unconscious state that forms a basis for further anesthetization <basal narcosis> <a basal anesthetic>
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