Pronunciation: (sē'nīl, -nil, sen'īl),[key] —adj. 1. showing a decline or deterioration of physical strength or mental functioning, esp. short-term memory and alertness, as a result of old age or disease.
2. of or belonging to old age or aged persons; gerontological; geriatric.
3. Physical Geog.(of topographical features) having been reduced by erosion to a featureless plain that stands everywhere at base level. Cf. peneplain.