adj

• (predicative) Returned or restored to a previous place or condition.
  "He was on vacation, but now he’s back."
• (comparable, phonetics) Pronounced with the highest part of the body of the tongue toward the back of the mouth, near the soft palate (most often describing a vowel).
  "The vowel of lot has a back vowel in most dialects of England."

adv

• (not comparable) To or in a previous condition or place.
  "He gave back the money."
• (not comparable) In a reciprocal manner; in return.
  "If you hurt me, I’ll hurt you back."
• (postpositive) Earlier, ago.
  "We met many years back."

noun

• The rear of the body, especially the part between the neck and the end of the spine and opposite the chest and belly.
  "Take the average black man and ask him that. She gotta pack much back."
• The rear of the body, especially the part between the neck and the end of the spine and opposite the chest and belly.
  "I still need to finish the back of your dress."
• The rear of the body, especially the part between the neck and the end of the spine and opposite the chest and belly.
  "Do thou but think / What ’tis to cram a maw or clothe a back / From such a filthy vice"
• That which is farthest away from the front.
  "Convenience and custom have familiarised us to the printed page being a little higher than the middle of the leaf, and to its having a little more margin at the fore edge than in the back."

verb

• (intransitive) To go in the reverse direction.
  "The train backed into the station."
• (transitive) To support.
  "I back you all the way."
• (nautical, of the wind) To change direction contrary to the normal pattern; that is, to shift anticlockwise in the northern hemisphere, or clockwise in the southern hemisphere.
• (nautical, of a square sail) To brace the yards so that the wind presses on the front of the sail, to slow the ship.