America/Canada is a slash pairing between the two North American nations: America (Alfred F. Jones) and Canada (Matthew Williams).
This pairing is referred to as AmeCan mostly (America and Canada). Although alternatively called UsCan, CanAme, and Americest.
This paring is based on geographic location of the two nations and the brotherly bond between the two. In addition, in the anime and manga Canada and America are often seen together.
This pairing is often associated with France/Britain.