Source : Webster's Revised Unabridged Dictionary (1913)

Idolum \I*do"lum\, Idolon \I*do"lon\, n.; pl. {Idola}. [L. See
   {Idol}; cf. {Eidolon}.]
   Appearance or image; a phantasm; a spectral image; also, a
   mental image or idea.