凝胶（英语：gel，来自拉丁语 gelu—寒冷、冰，或 gelatus—冻结、不可动）是一种固体的、类似果冻的材料。这种材料可以很柔软，也可以很坚硬。凝胶是一种充分稀释的交联系统，在稳定状态下没有流动性。 以重量计算，凝胶的主要成分是液体，但由于液体中的三维交联网络，凝胶在很多方面有着与固体相近的特性。

此概念常与“溶胶”相对（参见溶胶凝胶）。

IUPAC定义 凝胶：非流动性的胶态网络或者是贯穿其整个体积由流体膨胀聚合物网络[4]

注1：一种凝胶具有有限的，通常相当小，屈服应力。

注2：凝胶可以包含：

（ⅰ）共价聚合物网络，例如，通过交联聚合物链或由非线性聚合形成的网络;

A gel is a solid jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state. By weight, gels are mostly liquid, yet they behave like solids due to a three-dimensional cross-linked network within the liquid. It is the crosslinking within the fluid that gives a gel its structure (hardness) and contributes to the adhesive stick (tack). In this way gels are a dispersion of molecules of a liquid within a solid in which the solid is the continuous phase and the liquid is the discontinuous phase. The word gel was coined by 19th-century Scottish chemist Thomas Graham by clipping from gelatine.