The Collaborative International Dictionary of English v.0.48:

Say \Say\, v. t. [imp. & p. p. Said (s[e^]d), contracted from
   sayed; p. pr. & vb. n. Saying.] [OE. seggen, seyen, siggen,
   sayen, sayn, AS. secgan; akin to OS. seggian, D. zeggen, LG.
   seggen, OHG. sag[=e]n, G. sagen, Icel. segja, Sw. s[aum]ga,
   Dan. sige, Lith. sakyti; cf. OL. insece tell, relate, Gr.
   'e`nnepe (for 'en-sepe), 'e`spete. Cf. Saga, Saw a
   saying.]
   1. To utter or express in words; to tell; to speak; to
      declare; as, he said many wise things.
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            Arise, and say how thou camest here.  --Shak.
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   2. To repeat; to rehearse; to recite; to pronounce; as, to
      say a lesson.
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            Of my instruction hast thou nothing bated
            In what thou hadst to say?            --Shak.
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            After which shall be said or sung the following
            hymn.                                 --Bk. of Com.
                                                  Prayer.
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   3. To announce as a decision or opinion; to state positively;
      to assert; hence, to form an opinion upon; to be sure
      about; to be determined in mind as to.
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            But what it is, hard is to say.       --Milton.
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   4. To mention or suggest as an estimate, hypothesis, or
      approximation; hence, to suppose; -- in the imperative,
      followed sometimes by the subjunctive; as, he had, say
      fifty thousand dollars; the fox had run, say ten miles.
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            Say, for nonpayment that the debt should double,
            Is twenty hundred kisses such a trouble? --Shak.
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   It is said, or They say, it is commonly reported; it is
      rumored; people assert or maintain.

   That is to say, that is; in other words; otherwise.
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WordNet (r) 3.0 (2006):

that is to say
    adv 1: as follows [syn: namely, viz., that is to say, to
           wit, videlicet]