Concise Bible Dictionary:
One who makes a will or testament, introduced in Hebrews 9:16-17 in a parenthesis, showing that as a will is of force only after a man is dead, so Christ must have died for the blessings of the new covenant to be available.
Strong’s Dictionary of Greek Words:
Transliteration:
diatithemai
Phonic:
dee-at-ith’-em-ahee
Meaning:
middle voice from 1223 and 5087; to put apart, i.e. (figuratively) dispose (by assignment, compact, or bequest)
KJV Usage:
appoint, make, testator