The Nisnevich topology is a certain Grothendieck topology on the category of schemes which is finer than the Zariski topology but coarser than the étale topology. It retains many desirable properties from both topologies:
The Nisnevich cohomological dimension (and even the homotopy dimension) of a scheme is bounded by its Krull dimension? (like Zariski)
Fields have trivial shape for the Nisnevich topology (like Zariski)
Algebraic K-theory satisfies descent over the Nisnevich site – as is true for the Zariski site but not in full generality for the etale site, see at algebraic K-theory – Descent for more;
Any smooth closed pair $(Z,X)$ (i.e., $Z$ is a closed subscheme of $X$ and $X$ and $Z$ are both smooth over a common base scheme) is Nisnevich-locally isomorphic to $(\mathbb{A}^n,\mathbb{A}^{n+k})$ (like étale)
Pushforward along a finite morphism? is an exact functor on Nisnevich sheaves of abelian groups (like étale)
Nisnevich cohomology can be computed using Čech cohomology (like étale)
The Nisnevich topology plays a central rôle in motivic homotopy theory.
An family of morphisms of Noetherian schemes $\{p_i:V_i\to U\}$ is a Nisnevich cover if each $p_i$ is an étale map and if every field-valued point $Spec k\to U$ lifts to one of the $V_i$. This is a pretopology on the category of Noetherian schemes, and the associated topology is the Nisnevich topology.
The Nisnevich site over a Noetherian scheme $S$ usually refers to the site given by the category of smooth schemes of finite type over $S$ equipped with the Nisnevich topology. The small Nisnevich site of $S$ is the subsite consisting of étale $S$-schemes.
For a general affine scheme $X$, one defines a sieve $S$ on $X$ to be a covering sieve for the Nisnevich topology if there exist a Noetherian affine scheme $Y$, a morphism $f: X\to Y$, and a Nisnevich covering sieve $T$ on $Y$ such that $f^\ast(T)\subset S$. On an arbitrary scheme $X$, a sieve $S$ is a Nisnevich covering sieve if there exists an open cover $\{U_i\to X\}$ by affine schemes such that $S_{/U_i}$ is a Nisnevich covering sieve on $U_i$ for all $i$.
Let $Et/S$ be the category of étale schemes of finite type over a Noetherian scheme $S$. An (∞,1)-presheaf $F$ on $Et/S$ is said to satisfy Nisnevich excision if the following conditions hold:
is an (∞,1)-pullback square. Intuitively, this says that the space of sections of $F$ over $X$ with support in $Z$ (i.e., the homotopy fiber of $F(X) \to F(X-Z)$) does not depend on $X$. This is Definition 2.5 in DAG XI.
An (∞,1)-presheaf on $Et/S$ is an (∞,1)-sheaf for the Nisnevich topology if and only if it satisifes Nisnevich excision.
This is Morel-Voevosky, Prop. 1.16 or DAG XI, Thm. 2.9.
If $S$ is a Noetherian scheme of finite Krull dimension?, then the (∞,1)-topos of (∞,1)-sheaves on the small Nisnevich site of $S$ has homotopy dimension $\leq\dim(S)$.
This is DAG XI, Theorem 2.24. As a consequence, Postnikov towers are convergent in the (∞,1)-topos of (∞,1)-sheaves on the Nisnevich site over $S$, and in particular that (∞,1)-topos is hypercomplete.
fpqc-site $\to$ fppf-site $\to$ syntomic site $\to$ étale site $\to$ Nisnevich site $\to$ Zariski site
A quick overview is at the beginning of the talk slides
A detailed discussion is in section 3.1.1 of
or in the lecture notes
A self-contained account of the Nisnevich $(\infty,1)$-topos including the non-Noetherian case is in