nLab
dendrex

Idea

The notion of dendrex is the operadic analog of the notion of simplex.

Definition

For $X$ a dendroidal set and $T$ a tree, a $T$-shaped dendrex of $X$ is a morphism

$$\Omega [T]\to X,$$\Omega[T] \to X \,,

hence equivalently, by the Yoneda lemma, an element of the set $X(T)$.