(CH20)n

from The Collaborative International Dictionary of English v.0.48

paraformaldehyde \par`a*form*al"de*hyde\, n. [Pref. para- +
   formaldehyde.] (Chem.)
   A polymer of formaldehyde obtained as a white crystalline
   substance by concentration of a formaldehyde solution.
   Formula {(CH20)n}. It dissolves in hot water with the release
   of formaldehyde, and is used as a disinfectant.
   [1913 Webster +PJC]