  Cosmological constant can always be considered as the on-shell value of a top
form in gravitational theories. The top form is field strength of a gauge
field, and the theory enjoys a gauge symmetry. We show that cosmological
constant is the charge of global part of the gauge symmetry, and is conserved
irrespective of the dynamics of the metric and other fields. In addition, we
introduce its conjugate chemical potential, and prove the generalized first law
of thermodynamics which includes variation of cosmological constant as a
conserved charge. We discuss how our new term in the first law is related to
the volume-pressure term. In parallel with the seminal Wald entropy, this
analysis suggests that pressure can also be considered as a conserved charge.
