The Collaborative International Dictionary of English v.0.48:

malignant tumor \malignant tumor\ n. (Med.),
   Same as malignancy[4].

   Syn: malignancy, malignance, malignant tumor, metastatic
        tumor.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Tumor \Tu"mor\ (t[=u]"m[~e]r), n. [L., fr. tumere to swell: cf.
   F. tume['u]r. See Tumid.]
   1. (Med.) A morbid swelling, prominence, or growth, on any
      part of the body; especially, a growth produced by
      deposition of new tissue; a neoplasm.
      [1913 Webster]

   2. Affected pomp; bombast; swelling words or expressions;
      false magnificence or sublimity. [R.]
      [1913 Webster]

            Better, however, to be a flippant, than, by a
            revolting form of tumor and perplexity, to lead men
            into habits of intellect such as result from the
            modern vice of English style.         --De Quincey.
      [1913 Webster]

   Encysted tumor, a tumor which is inclosed in a membrane
      called a cyst, connected with the surrounding parts by the
      neighboring cellular substance.

   Fatty tumor. See under Fatty.

   Innocent tumor, or Benign tumor, one which does not of
      itself threaten life, and does not usually tend to recur
      after extirpation; a tumor which has not metastesized.

   Malignant tumor, a tumor which tends continually to spread,
      to become generalized in different parts of the body, and
      to recur after extirpation, and which, if left to itself,
      causes death.
      [1913 Webster]

WordNet (r) 3.0 (2006):

malignant tumor
    n 1: a tumor that is malignant and tends to spread to other
         parts of the body [syn: malignant tumor, malignant
         neoplasm, metastatic tumor]