1. To separate, to set apart from others; that which is so treated. 2. To free of chemical contaminants. 3. In psychoanalysis, to separate ideas, experiences, or memories from the affects pertaining to them. 4. In group psychotherapy, a patient to whom others in the group do not respond. 5. Viable organisms separated on a single occasion from a sample taken from a host or culture system. 6. A population that for geographic, linguistic, cultural, social, religious, or other reasons is subject to little or no gene flow.