Question

Isotherms are the graphs drawn at constant __________.

Answer 1

An isothermal process is a change of a system in which the temperature remains constant:

$\Delta T=0$

Typically this occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change occurs slowly enough to allow the system to adjust continually to the temperature of the reservoir through heat exchange. So in the isotherms graph, the temperature is kept constant.