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K-essence is a cosmological scalar-field mechanism in which noncanonical kinetic dynamics supply negative pressure or accelerated expansion. Its action is a particular use of a P(X, phi) scalar field theory; that general action also includes theories without kinetic dark energy.

A $P(X,\varphi )$ scalar theory has Lagrangian density $P$ depending on the field and $X=-g^{\mu \nu }{\partial }_{\mu }\varphi {\partial }_{\nu }\varphi /2$. It includes canonical scalar theory and models with noncanonical kinetic interactions.

A k-essence model uses noncanonical kinetic terms for cosmological acceleration; a general P(X, phi) scalar field theory need not have that application.