symmetric monoidal (∞,1)-category of spectra
The associahedra or Stasheff polytopes $\{K_n\}$ are CW complexes that naturally arrange themselves into an topological operad that resolves the standard associative operad: an A-infinity-operad.
The vertices of $K_n$ correspond to ways in which one can bracket a product of $n$ variables. The edges correspond to rebracketings, the faces relate different sequences of rebracketings that lead to the same result, and so on.
The associahedra were introduced by Jim Stasheff in order to describe topological spaces equipped with a multiplication operation that is associative up to every higher coherent homotopy.
Here is the rough idea, copied, for the moment, verbatim from Markl94 p. 26 (for more details see references below):
For $n \geq 1$ the associahedron $K_n$ is an $(n-2)$-dimensional polyhedron whose $i$-dimensional cells are, for $0 \leq i \leq n-2$, indexed by all (meaningful) insertions of $(n-i-2)$ pairs of brackets between $n$ independent indeterminates, with suitably defined incidence maps.
Simplicially subdivided associahedra (complete with simplicial operadic structure) can be presented efficiently in terms of an abstract bar construction. Let $\mathcal{O}: Set/\mathbb{N} \to Set/\mathbb{N}$ be the monad which takes a graded set $X$ to the non-permutative non-unital operad freely generated by $X$, with monad multiplication denoted $m: \mathcal{O}\mathcal{O} \to \mathcal{O}$. Let $t_+$ be the graded set $\{X_n\}_{n \geq 0}$ that is empty for $n = 0, 1$ and terminal for $n \geq 2$; this carries a unique non-unital non-permutative operad structure, via a structure map $\alpha: \mathcal{O}t_+ \to t_+$. The bar construction $B(\mathcal{O}, \mathcal{O}, t_+)$ is an (augmented) simplicial graded set (an object in $Set^{\Delta^{op} \times \mathbb{N}}$) whose face maps take the form
Intuitively, the (graded set of) $0$-cells $\mathcal{O}t_+$ consists of planar trees where each inner node has two or more incoming edges, with trees graded by number of leaves; the extreme points are binary trees [corresponding to complete binary bracketings of words], whereas other trees are barycenters of higher-dimensional faces of Stasheff polytopes. The construction $B(\mathcal{O}, \mathcal{O}, t_+)$ carries a simplicial (non-permutative non-unital) operad structure, where the geometric realization of the simplicial set at grade (or arity?) $n$ defines the barycentric subdivision of the Stasheff polytope $K_n$. As the operad structure on $B(\mathcal{O}, \mathcal{O}, t_+)$ is expressed in finite product logic and geometric realization preserves finite products, the (simplicially subdivided) associahedra form in this way the components of a topological operad.
$K_1$ is the empty set, a degenerate case not usually considered.
$K_2$ is simply the shape of a binary operation:
which we interpret here as a single point.
$K_3$ is the shape of the usual associator or associative law
consisting of a single interval.
$K_4$ The fourth associahedron $K_4$ is the pentagon which expresses the different ways a product of four elements may be bracketed
One can also think of this as the top-level structure of the 4th oriental. This controls in particular the pentagon identity in the definition of monoidal category, as discussed there.
(image from the Wikimedia Commons)
A template which can be cut out and assembled into a $K_5$ can be found here.
Rotatable illustrations of some Stasheff polyhedra can be found at
This is part of a larger website Petites pages sur divers sujets which contains illusrations of other polyedra, too.
The above list shows that the first few Stasheff polytopes are nothing but the first few orientals. This doesn’t remain true as $n$ increases. The orientals are free strict omega-categories on simplexes as parity complexes. This means that certain interchange cells (e.g., Gray tensorators) show up as thin in the oriental description.
The first place this happens is the sixth oriental: where there are three tensorator squares and six pentagons in Stasheff’s $K_6$, the corresponding tensorator squares coming from $O(6)$ are collapsed.
It was when Todd Trimble made this point to Ross Street that Street began to think about using associahedra to define weak n-categories.
There is a categorification of associahedra discussed in
The original articles that define associahedra and in which the operad $K$ that gives $A(\infty)$-topological spaces is implicit are
Jim Stasheff, Homotopy associativity of H-spaces I, Trans. Amer. Math. Soc. 108 (1963), 275–312. (web)
Jim Stasheff, Homotopy associativity of H-spaces II, Trans. Amer. Math. Soc. 108 (1963), 293–312. (web)
A textbook discussion (slightly modified) is in section 1.6 of the book
Further explanations and references are collected at
Alexander Postnikov, Permutohedra, associahedra and beyond, math.CO/0507163 pdf