The Greek verb hagiazō means to sanctify, to make holy, to set apart for sacred use, or to treat as holy. It is the verbal form of hagios (holy) and occurs 28 times in the New Testament. It describes both the definitive act of God setting believers apart (positional sanctification) and the ongoing process of becoming holy (progressive sanctification).
Hagiazō is one of the most theologically loaded words in the NT. John 17:17 — 'Sanctify them by the truth; your word is truth' — is Jesus's high-priestly prayer that the Father set the disciples apart through encounter with divine truth. Hebrews uses the word repeatedly for Jesus's sacrificial work: 'by one sacrifice he has made perfect forever those who are being made holy' (Hebrews 10:14). Paul declares believers were 'sanctified' at conversion (1 Corinthians 6:11) — a completed act — while also urging them to pursue holiness. This tension (already-sanctified, being-sanctified) is the engine of Christian ethics: you are holy, therefore live holy.