Is it ensure, insure, or assure?
1580s, "censorious," from critic + -al (1). Meaning "pertaining to criticism" is from 1741; medical sense is from c.1600; meaning "of the nature of a crisis" is from 1640s; that of "crucial" is from 1841, from the "decisive" sense in Latin criticus. Related: Criticality (1756; in the nuclear sense, 1950); critically (1650s, "accurately;" 1815, "in a critical situation"). In nuclear science, critical mass is attested from 1940.
critical crit·i·cal (krĭt'ĭ-kəl)
Of or relating to a medical crisis.
Being or relating to a grave physical condition especially of a patient.
Of or relating to the value of a measurement, such as temperature, at which an abrupt change in a chemical of physical quality, property, or state occurs.