大冰期（英语：Ice Age），又称“冰河时期”或“冰川时期”，是指地球大气和地表长期低温导致极地和山地冰盖大幅扩展甚至覆盖整个大陆的时期。相邻的大冰期之间的气候比较温暖的时期，称为“大间冰期”。大冰期内部又分为若干次冰期（glacial period、glacials或glaciations）与间冰期（interglacials）。

从冰川学的角度，南北半球出现大范围冰盖的时期即可视作大冰期。鉴于格陵兰和南北极大范围冰盖的存在，当今的地球仍处在始于260万年前更新世的第四纪大冰期的一次间冰期中，且尚无迹象表明地球正在走出这次大冰期。

An ice age is a period of long-term reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Within a long-term ice age, individual pulses of cold climate are termed "glacial periods" (or alternatively "glacials" or "glaciations" or colloquially as "ice age"), and intermittent warm periods are called "interglacials". Glaciologically, ice age implies the presence of extensive ice sheets in the northern and southern hemispheres. By this definition, we are in an interglacial period—the Holocene—of the ice age that began 2.6 million years ago at the start of the Pleistocene epoch, because the Greenland, Arctic, and Antarctic ice sheets still exist.