From BiblePortal Wikipedia

Vine's Expository Dictionary of OT Words [1]

A. Adverb.

'Achar ( אַחַר , Strong'S #310), “behind; after(wards).” A cognate of this word occurs in Ugaritic. 'Achar appears about 713 times in biblical Hebrew and in all periods. One adverbial use of 'achar has a local-spatial emphasis that means “behind”: “The singers went before, the players on instruments followed after …” (Ps. 68:25). Another adverbial usage has a temporal emphasis that can mean “afterwards”: “And I will fetch a morsel of bread, and comfort ye your hearts; after that ye shall pass on …” (Gen. 18:5).

B. Preposition.

'achar ( אַחַר , Strong'S #310), “behind; after.” 'Achar as a preposition can have a local-spatial significance, such as “behind”: “And the man said, They are departed hence; for I heard them say, Let us go to Dothan” (Gen. 37:17). As such, it can mean “follow after”: “And also the king that reigneth over you [will] continue following the Lord your God” (1 Sam. 12:14). 'Achar can signify “after” with a temporal emphasis: “And Noah lived after the flood three hundred and fifty years” (Gen. 9:28, the first biblical occurrence of the word). This same emphasis may occur when 'achar appears in the plural (cf. Gen. 19:6—localspatial; Gen. 17:8—temporal).

C. Conjunction.

'Achar ( אַחַר , Strong'S #310), “after.” 'Achar may be a conjunction, “after,” with a temporal emphasis: “And the days of Adam after he had begotten Seth were eight hundred years …” (Gen. 5:4).

King James Dictionary [2]

BEHIND, prep.

1. At the back of another as, to ride behind a horseman. 2. On the back part, at any distance in the rear as, to walk behind another. 3. Remaining left after the departure of another, whether by removing to a distance, or by death as, a man leaves his servant behind him, or his estate at his decease. 4. Left at a distance, in progress or improvement as, one student is behind another in mathematics. 5. Inferior to another in dignity and excellence.

For I suppose I was not a whit behind the very chiefest apostles.  2 Corinthians 11

6. On the side opposite to that which fronts a person on the other side as behind a bed behind a hill behind a house, tree, or rock.

Behind the back, in scripture, signifies,out of notice, or regard overlooked disregarded.

They cast thy laws behind their backs.  Nehemiah 19;  Isaiah 38

BEHIND, adv. be and hind. Out of sight not produced, or exhibited to view remaining as, we know no what evidence is behind.

1. Backwards on the back-part as, to look behind. 2. Past in the progress of time. 3. Future, or remaining to be endured.

And fill up that which is behind of the afflictions of Christ in my flesh.  Colossians 1

4. Remaining after a payment unpaid as, there is a large sum behind. 5. Remaining after the departure of as, he departed and left us behind.

Webster's Dictionary [3]

(1): (adv.) After the departure of another; as, to stay behind.

(2): (n.) The backside; the rump.

(3): (adv.) At the back part; in the rear.

(4): (a.) Left a distance by, in progress of improvement Hence: Inferior to in dignity, rank, knowledge, or excellence, or in any achievement.

(5): (adv.) Toward the back part or rear; backward; as, to look behind.

(6): (adv.) Backward in time or order of succession; past.

(7): (a.) On the side opposite the front or nearest part; on the back side of; at the back of; on the other side of; as, behind a door; behind a hill.

(8): (a.) Left after the departure of, whether this be by removing to a distance or by death.

(9): (adv.) Not yet brought forward, produced, or exhibited to view; out of sight; remaining.