The Collaborative International Dictionary of English v.0.48:

Tab \Tab\, n. [Etymol. uncertain.]
   1. The flap or latchet of a shoe fastened with a string or a
      buckle.
      [1913 Webster]

   2. A tag. See Tag, 2.
      [1913 Webster]

   3. A loop for pulling or lifting something.
      [1913 Webster]

   4. A border of lace or other material, worn on the inner
      front edge of ladies' bonnets.
      [1913 Webster]

   5. A loose pendent part of a lady's garment; esp., one of a
      series of pendent squares forming an edge or border.
      [1913 Webster]

   6. a small projecting piece of a file folder, file card, or
      similar sheet used in a filing system, on which a notation
      is written to permit convenient search for the folder,
      card, etc.
      [PJC]

   7. a bill or check for some purchase, as in a restaurant; as,
      the salesman will pick up the tab.
      [PJC]

   8. a key on a typewriter or computer keyboard which advances
      the carriage or curser to the next (preset) tab position;
      -- used especially to type or print text or numbers in
      columns.
      [PJC]

WordNet (r) 3.0 (2006):

tab
    n 1: the bill in a restaurant; "he asked the waiter for the
         check" [syn: check, chit, tab]
    2: sensationalist journalism [syn: yellow journalism,
       tabloid, tab]
    3: the key on a typewriter or a word processor that causes a
       tabulation [syn: tab key, tab]
    4: a short strip of material attached to or projecting from
       something in order to facilitate opening or identifying or
       handling it; "pull the tab to open the can"; "files with a
       red tab will be stored separately"; "the collar has a tab
       with a button hole"; "the filing cards were organized by
       cards having indexed tabs"
    5: a dose of medicine in the form of a small pellet [syn:
       pill, lozenge, tablet, tab]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "tab":
   a reckoning of, account, account of, act drop, afterpart,
   afterpiece, amount, appoint, asbestos, asbestos board, assign,
   backdrop, batten, bilge, bill, bill of account, bill of lading,
   blain, bleb, blister, blob, border, boss, bow, brand, bubble, bulb,
   bulge, bulla, bump, bunch, burl, button, cahot, charge, check,
   check of, chine, cloth, clump, condyle, convex, cost, coulisse,
   count of, counterweight, curtain, curtain board, cyclorama, damage,
   decor, designate, dowel, drop, drop curtain, dun, eagle eye, ear,
   earmark, expenditure, expense, eye, figure, fire curtain, flag,
   flange, flap, flat, flipper, gall, gnarl, hallmark, handle,
   hanging, hill, hump, hunch, invoice, itemized bill, jog, joggle,
   knob, knot, knur, knurl, label, lappet, lip, loop, lump, manifest,
   mole, mountain, name, nevus, nominate, nub, nubbin, nubble, ordain,
   ordinate, papilloma, peg, price, price tag, queue, rag, rate,
   reckoning, rib, ridge, ring, scene, scenery, score, screen,
   scrutiny, seal, select, shoulder, side scene, spine, stage screw,
   stamp, statement, sticker, strap, stud, style, surveillance,
   tableau, tabs of, tag, tail, tailpiece, tally of, tariff, teaser,
   ticket, tormentor, track of, trail, trailer, train, transformation,
   transformation scene, tubercle, tubercule, verruca, vesicle, wake,
   wale, wart, watch, welt, wing, wingcut, woodcut



The Free On-line Dictionary of Computing (30 December 2018):

horizontal tabulation
\t
HT
TAB

    (tab, Control-I, HT, ASCII 9) A character which
   when displayed or printed causes the following character to be
   placed at the next "tabstop" - the column whose number is a
   multiple of the current tab width.  Commonly (especially in
   Unix(?)) the tab width is eight, so, counting from the left
   margin (column zero), the tab stops are at columns 8, 16, 24,
   up to the width of the screen or page.

   A tab width of four or two is often preferred when indenting
   program source code to conserve indentation.

   Represented as "\t" in C, Unix, and derivatives.

   (1999-07-05)