early 15c., tenacite, "quality of holding firmly," from Old French ténacité (14c.) and directly from Latin tenacitas "an act of holding fast," from tenax (genitive tenacis) "holding fast, gripping, clingy; firm, steadfast," from tenere "to hold" (from PIE root *ten- "to stretch"). The PIE root produced two Latin verbs, tenere "to hold, grasp," and tendere "to stretch" (as in tend (v.1)), which perhaps is from an inflected form in the PIE verb. Both Latin verbs have past participle tentus.
late 14c., retencioun, "the keeping of fluid or secretions within the body," also "power of capacity," from Latin retentionem (nominative retentio) "a retaining, a holding back," noun of action from past-participle stem of retinere (see retain).
The mental sense of "remembrance, fact of retaining things in the mind" is from late 15c. As "act of retaining or holding as one's own," from 1610s; retention rate is by 1972.