纳米复合物是一种多相固态材料，其中一个相态至少有一边的尺寸小于100纳米. 纳米复合物通常是由因结构和化学性质不同而具有不同性质的一个基质与一个纳米级增强相组合而成。

纳米复合物的力学性质与传统的复合材料不同，因为纳米复合物的增强相具有很高的面积/体积比与长宽比。增强相一般是以颗粒状（矿物），层状（粘土）或者纤维状（碳纳米管）形态存在的。基质与增强相的界面面积非常大，通常比传统复合材料大一个数量级。大的界面面积意味着很少量的增强相可以对复合物整体的宏观性质产生影响。例如，复合材料中加入的碳纳米管可以增强电导率和热导率。加入其他纳米颗粒则可能改善材料的光学性质，介电性质，耐火性或者力学性质。

Nanocomposite is a multiphase solid material where one of the phases has one, two or three dimensions of less than 100 nanometers (nm), or structures having nano-scale repeat distances between the different phases that make up the material. In the broadest sense this definition can include porous media, colloids, gels and copolymers, but is more usually taken to mean the solid combination of a bulk matrix and nano-dimensional phase(s) differing in properties due to dissimilarities in structure and chemistry. The mechanical, electrical, thermal, optical, electrochemical, catalytic properties of the nanocomposite will differ markedly from that of the component materials. Size limits for these effects have been proposed, <5 nm for catalytic activity, <20 nm for making a hard magnetic material soft, <50 nm for refractive index changes, and <100 nm for achieving superparamagnetism, mechanical strengthening or restricting matrix dislocation movement.