equivalences in/of $(\infty,1)$-categories
The delooping of an object $A$ is, if it exists, a uniquely pointed object $\mathbf{B} A$ such that $A$ is the loop space object of $\mathbf{B} A$:
In particular, if $A = G$ is a group then its delooping
in the context Top is the classifying space $\mathcal{B}G$
in the context ∞-Grpd is the one-object groupoid $\mathbf{B}G$.
Under the homotopy hypothesis these two objects are identified: the geometric realization of the groupoid $\mathbf{B}G$ is the classifying space $\mathcal{B}G$:
Loop space objects are defined in any (∞,1)-category $\mathbf{C}$ with homotopy pullbacks: for $X$ any pointed object of $\mathbf{C}$ with point ${*} \to X$, its loop space object is the homotopy pullback $\Omega X$ of this point along itself:
Conversely, if $A$ is given and a homotopy pullback diagram
exists, with the point ${*} \to \mathbf{B} A$ being essentially unique, by the above $A$ has been realized as the loop space object of $\mathbf{B} A$
and we say that $\mathbf{B} A$ is the delooping of $A$.
See the section delooping at groupoid object in an (∞,1)-category for more.
If $\mathbf{C}$ is even a stable (∞,1)-category then all deloopings exist and are then also denoted $\Sigma A$ and called the suspension of $A$.
In section 6.1.3 of
a definition of groupoid object in an (infinity,1)-category $\mathbf{C}$ is given as a homotopy simplicial object, i.e. a (infinity,1)-functor
satisfying certain conditions (prop. 6.1.2.6) which are such that if $C_0 = {*}$ is the point we have an internal group in a homotopical sense, given by an object $C_1$ equipped with a coherently associative multiplication operation $C_1 \times C_1 \to C_1$ generalizing that of Stasheff H-space from the $(\infty,1)$-category Top to arbitrary $(\infty,1)$-categories.
Lurie calls the groupoid object $C$ an effective groupoid object in an (infinity,1)-category precisely if it arises as the delooping, in the above sense, of some object $\mathbf{B}C$.
One of the characterizing properties of an (infinity,1)-topos is that every groupoid object in it is effective.
This is the analog of Stasheff’s classical result about H-spaces.
See the remark at the very end of section 6.1.2 in HTT.
For $C =$ Top the (infinity,1)-category of topological spaces, a space is deloopable if it is an A-infinity-space and hence homotopy equivalent to a loop space.
Let $G$ be a group regarded as a discrete groupoid in the (∞,1)-topos ∞Grpd of ∞-groupoids.
Then $\mathbf{B} G$ exists and is, up to equivalence, the groupoid
with a single object $\bullet$,
with $Hom_{\mathbf{B} G}(\bullet, \bullet) = G$, or equivalently $Aut_{\mathbf{B}G}(\bullet) = G$,
and with composition of morphisms in $\mathbf{B} G$ being given by the product operation in the group.
More informally but more suggestively we may write
or
to emphasize that there is really only a single object.
Notice how the homotopy pullback works in this simple case:
the universal 2-cell $\eta$
filling this 2-limit diagram is the natural transformation from the constant functor
to itself, whose component map
is just the identity map, using that $Obj(G) = G$ and $Mor(\mathbf{B}G) = G$.
loop space object, free loop space object,
delooping