Allophone is a word in the Canadian English dialect used to descibe someone whose mother tongue is neither English nor French. Apparently first used by statisticians in the employ of the Quebec government, it has become a standard in both census work and in the Canadian media; it is also in common use by the Canadian people.
The origin is Greek, with the first constituent of the word (allo) coming from the Greek for other and the second part (phone) coming from the Greek for sound or voice. Very simply, allophone means "other language" in this context.
Canadian census reports therefore show three categories languages in general: Anglophone (English speaker); Francophone (French speaker); and Allophone. However, respondants choose the specific language they spoke first and census data is available for specific languages.
Personal note
I am an anglophone, with my Mom being anglophone and Dad francophone. My Dad's Mom was allophone, my Dad's Dad francophone. My Mom's Dad was anglophone and my Mom's Mom francophone. My girlfriend is allophone, as are her parents. Confused yet?
Many of my friends come from all three segments, even if the language they use today is different (my girlfriend may be officially allophone but uses English more than Portuguese today.)
Sources:
http://www.cogsci.indiana.edu/farg/harry/lan/grkmore1.htm (About the Greek Language)
http://www2.marianopolis.edu/quebechistory/events/allos.htm (Quebec History, Marianopolis College, Montreal)