uses 
 RichEdit; 
  
procedure TForm1.RichEdit1MouseMove(Sender: TObject; Shift: TShiftState; 
  X, Y: Integer); 
var 
  iCharIndex, iLineIndex, iCharOffset, i, j: Integer; 
  Pt: TPoint; 
  s: string; 
begin 
  with TRichEdit(Sender) do 
  begin 
    Pt := Point(X, Y); 
    // Get Character Index from word under the cursor 
    iCharIndex := Perform(Messages.EM_CHARFROMPOS, 0, Integer(@Pt)); 
    if iCharIndex < 0 then Exit; 
    // Get line Index 
    iLineIndex  := Perform(EM_EXLINEFROMCHAR, 0, iCharIndex); 
    iCharOffset := iCharIndex - Perform(EM_LINEINDEX, iLineIndex, 0); 
    if Lines.Count - 1 < iLineIndex then Exit; 
    // store the current line in a variable 
    s := Lines[iLineIndex]; 
    // Search the beginning of the word 
    i := iCharOffset + 1; 
    while (i > 0) and (s[i] <> ' ') do Dec(i); 
    // Search the end of the word 
    j := iCharOffset + 1; 
    while (j <= Length(s)) and (s[j] <> ' ') do Inc(j); 
    // Display Text under Cursor 
    Caption := Copy(s, i, j - i); 
  end; 
end;