group cohomology, nonabelian group cohomology, Lie group cohomology
cohomology with constant coefficients / with a local system of coefficients
differential cohomology
Hochschild homology may be understood as the cohomology of free loop space objects (as described there). These free loop space objects are canonically equipped with a circle group-action that rotates the loops. Cyclic homology is the corresponding $S^1$-equivariant cohomology of free loop space objects.
Like Hochschild homology, cyclic homology is an additive invariant of dg-categories or stable infinity-categories, in the sense of noncommutative motives. It also admits a Dennis trace map from algebraic K-theory, and has been successful in allowing computations of the latter.
There are several definitions for the cyclic homology of an associative algebra $A$ (over a commutative ring $k$). Alain Connes originally defined cyclic homology over fields of characteristic zero, as the homology groups of a cyclic variant of the chain complex computing Hochschild homology. Jean-Louis Loday and Daniel Quillen gave a definition via a certain double complex (for arbitrary commutative rings). Connes gave another definition by associating to $A$ a cyclic vector space $A^\sharp$, and showing that the cyclic homology of $A$ may be computed as via Ext-groups $Ext^*(A^\sharp, k^\sharp)$. A fourth definition was given by Christian Kassel, who showed that the cyclic homology groups may be computed as the homology groups of a certain mixed complex associated to $A$.
Following Alexandre Grothendieck, Charles Weibel gave a definition of cyclic homology (and Hochschild homology) for schemes, using hypercohomology. On the other hand, the definition of Christian Kassel via mixed complexes was extended by Bernhard Keller to linear categories and dg-categories, and he showed that the cyclic homology of the dg-category of perfect complexes on a (nice) scheme $X$ coincides with the cyclic homology of $X$ in the sense of Weibel.
There are closely related variants called periodic cyclic homology? and negative cyclic homology?. There is a version for ring spectra called topological cyclic homology.
Let $A$ be an associative algebra over a ring $k$. Write $C_\bullet(A,A)$ for the Hochschild homology chain complex of $A$ with coefficients in $A$.
For each $n \in \mathbb{N}$ let $\lambda : C_n(A,A) \to C_n(A,A)$ be the $k$-linear map that cyclically permutes the elements and introduces a sign:
The cyclic homology complex $C^\lambda_\bullet(A)$ of $A$ is the quotient of the Hochschild homology complex of $A$ by cyclic permutations:
The homology of the cyclic complex, denoted
is called the cyclic homology of $A$.
The cyclic cohomology groups of $A$ Are the cohomology groups of the dual cochain complex, denoted $HC^n(A)$.
If $I\subset A$ is an ideal, then the relative cyclic homology groups $HC_n(A,I)$ are the homology groups of the complex $C_\bullet(A,I) = ker(C_\bullet(A)\to C_\bullet(A/I))$.
Foundational articles:
Monographs:
Quick lecture notes:
D. Kaledin, Tokyo lectures “Homological methods in non-commutative geometry”, (pdf, TeX); and related but different Seoul lectures
Masoud Khalkhali, A short survey of cyclic cohomology, arxiv/1008.1212
Some modern treatments:
Bernhard Keller, On the cyclic homology of ringed spaces and schemes, Doc. Math. J. DMV 3 (1998), 231-259, pdf.
Bernhard Keller, On the cyclic homology of exact categories, Journal of Pure and Applied Algebra 136 (1999), 1-56, pdf.
Bernhard Keller, Invariance and Localization for Cyclic Homology of DG algebras, Journal of Pure and Applied Algebra, 123 (1998), 223-273, pdf.
Charles Weibel, Cyclic homology for schemes, Proceedings of the AMS, 124 (1996), 1655-1662, web.
D. Kaledin, Cyclic homology with coefficients, math.KT/0702068, to appear in Yu. Manin’s 70th anniversary volume.
E. Getzler, M. Kapranov, Cyclic operads and cyclic homology, in: “Geometry, Topology and Physics for R. Bott”, ed. S.-T. Yau, p. 167-201, International Press, Cambridge MA, 1995, pdf
Teimuraz Pirashvili, Birgit Richter, Hochschild and cyclic homology via functor homology, K-Theory 25 (2002), no. 1, 39–49, MR2003c:16011, doi
Jolanta Słomińska, Decompositions of the category of noncommutative sets and Hochschild and cyclic homology, Cent. Eur. J. Math. 1 (2003), no. 3, 327–331, MR2004f:16011, doi
Some influential original references from 1980s: