1) to attend to carefully, take care of
1a) to guard
1b) metaphorically to keep, one in the state in which he is
1c) to observe
1d) to reserve: to undergo something
Part of Speech: verb
Citing in TDNT: 8:140, 1174
From τηρός teros (a watch; perhaps akin to G2334); to guard (from loss or injury, properly by keeping the eye upon; and thus differing from G5442, which is properly to prevent escaping; and from G2892, which implies a fortress or full military lines of apparatus), that is, to note (a prophecy; figuratively to fulfil a command); by implication to detain (in custody; figuratively to maintain); by extension to withhold (for personal ends; figuratively to keep unmarried): - hold fast, keep (-er), (ob-, pre-, re) serve, watch.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root τηρέω; click on the second column to search for that grammatical form of the root τηρέω; click on the third column to search for that word and grammatical form; click on the total to search for the root τηρέω.