One definition of temperature I have seen is
$$T= \left(\frac{\partial E}{\partial S}\right)_N.$$
While $E, S, N$ each has clear meaning (internal energy, entropy, number of particles), I don't know how to interpret the subscript $N$.
Do I take an average among all particles or is there any other meaning?
The subscript $N$ on the partial derivative means that $N$ is held constant.