The Collaborative International Dictionary of English v.0.48:

Marginal \Mar"gin*al\, a. [Cf. F. marginal.]
   1. Of or pertaining to a margin.
      [1913 Webster]

   2. Written or printed in the margin; as, a marginal note or
      gloss.
      [1913 Webster]

   3. At the lower limit; barely sufficient; as, of marginal
      utility.
      [PJC] marginal cost

WordNet (r) 3.0 (2006):

marginal
    adj 1: at or constituting a border or edge; "the marginal strip
           of beach" [syn: fringy, marginal]
    2: of questionable or minimal quality; "borderline grades";
       "marginal writing ability" [syn: borderline, marginal]
    3: just barely adequate or within a lower limit; "a bare
       majority"; "a marginal victory" [syn: bare(a), marginal]
    4: producing at a rate that barely covers production costs;
       "marginal industries"

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "marginal":
   bordering, borderline, boundary, bounding, coastal, determinant,
   determinative, determining, disputable, doubtful, dubious, extreme,
   fringing, frontier, inferior, infinitesimal, insignificant, limbic,
   liminal, limit, limiting, littoral, low-priority, minimal,
   negligible, no great shakes, of no account, of no consequence,
   of no matter, of no significance, on the edge, questionable,
   rimming, secondary, skirting, slight, small, terminal, threshold,
   tiny, unimportant



The Jargon File (version 4.4.7, 29 Dec 2003):

marginal
 adj.

    [common]

    1. [techspeak] An extremely small change. ?A marginal increase in core
    can decrease GC time drastically.? In everyday terms, this means that it
    is a lot easier to clean off your desk if you have a spare place to put
    some of the junk while you sort through it.

    2. Of little merit. ?This proposed new feature seems rather marginal to me.
    ?

    3. Of extremely small probability of winning. ?The power supply was
    rather marginal anyway; no wonder it fried.?


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

marginal

    1. Extremely small.  "A marginal increase in core
   can decrease GC time drastically."  In everyday terms, this
   means that it is a lot easier to clean off your desk if you
   have a spare place to put some of the junk while you sort
   through it.

   2. Of extremely small merit.  "This proposed new feature seems
   rather marginal to me."

   3. Of extremely small probability of winning.  "The power
   supply was rather marginal anyway; no wonder it fried."

   [Jargon File]

   (1994-10-21)