Hagios

ἅγιος hágios; fem. hagía, neut. hágion (39), adj. from hágos (n.f.), any matter of religious awe, expiation, sacrifice. Holy, set apart, sanctified, consecrated, saint. It has a common root, hág-, with hagnós (53), chaste, pure. Its fundamental idea is separation, consecration, devotion to the service of Deity, sharing in God’s purity and abstaining from earth’s defilement.

(I) Pure, clean, ceremonially or morally clean, including the idea of deserved respect, reverence.

Spiros Zodhiates, The Complete Word Study Dictionary: New Testament (Chattanooga, TN: AMG Publishers, 2000).

The central idea is someone separated to God, and consequently living in a manner that favorably reflects His character and characteristics.