You're definitely on the right track. A tank is best defined by doctrinal use, not by physical characteristics because many things have been officially used as

You're definitely on the right track. A tank is best defined by doctrinal use, not by physical characteristics because many things have been officially used as tanks that would not fit a simple definition. The definition most people would come up with would best describe an Armored Fighting Vehicle (AFV). Tanks are a type of AFV but not all AFVs are tanks.