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In an action functional on a space of pseudo-Riemannian manifolds – such as the Einstein-Hilbert action functional for gravity – a cosmological constant is a term proportional to the volume
where $\lambda \in \mathbb{R}$ is the cosmological constant .
For instance pure Einstein-Hilbert gravity with cosmological constant (and not other fields) is given by the functional
Generically it happens that one considers action functionals where $\lambda$ is in fact not a constant, but a function of other fields $\phi$ on $X$.
In this context those solutions to the Euler-Lagrange equations are of interest in which $\lambda(\phi)$ happens to be exactly or approximately constant. Many such models of not-really-constant-but-effectively-constant terms proportional to the volume are being proposed and considered in attempts to explain observed or speculated dynamics of the cosmos.
See in particular at FRW model for the role of the cosmological constant in homogeneous and isotropic models as in the standard model of cosmology. In that context the cosmological constant is also called the dark energy (density), which makes up about 70% of the energy density of the observable universe (the rest being dark matter) and a comparatively little bit of baryonic matter.
A good review is in