Good question. As specialist house sign makers we usually call our products house signs regardless of what materials they are made from. According to oxforddictionaries.com 'sign' is defined as ' A notice on public display that gives information or instructions in a written or symbolic form'. A 'plaque' is defined as 'An ornamental tablet, typically of metal, porcelain, or wood, that is fixed to a wall or other surface in commemoration of a person or event' and a 'plate' is 'A small, flat piece of metal or other material bearing a name or inscription and attached to a door or other object.'
We have found that most people in the UK call signs with house numbers 'house number plates' as they tend to associate the words 'plate' and 'numbers' as there is a common term 'car number plate' which rolls naturally of the tongue.
Many customers refer to our hand painted signs as 'plaques' although technically the signs don't usually commemorate any event. We think this is because a 'plaque' gives an air of exclusiveness
Most UK customer do call house name signs and address signs as 'house signs' but american customers call them 'yard signs' or 'address markers'.
In conclusion nowadays it does not really matter what you call a house sign. As long as it is large enough for people to read it from the road it will be doing it's job.