What is two batteries in a circuit called?

If the batteries are connected from + (positive) to - (negative) then the batteries are connected in "series". If the batteries are AA (1.5v each), then the resulting voltage of the two connected in series will be 3.0v.

If the batteries are connected + to + and - to -, then they are connected in "parallel" and for the same batteries the voltage would be 1.5v, but would last twice as long as one battery by itself.

A truck battery system is two 12v batteries connected in series (+ to -) to give the truck a running power of 24v.