It's not defined. Paragraph 9 of the 1997 commentaries says “small 'facilitation' payments do not constitute payments to obtain or retain business or other improper advantage”. That is the reason I said earlier that it's not clear whether or not those payments would be prohibited by the central prohibition in the CFPOA. And the OECD guidance as it currently stands is to encourage countries to take measures to encourage companies to ban them.
On June 13th, 2013. See this statement in context.