激进主义是指对现存组织、社会状态或运作方式怀有强烈的不满，从而产生否定的观念，并迫切寻求对现状从根本上进行剧烈的变革。激进主义也分为左右两派，如有主张恢复到过去的伊斯兰原教旨主义，也有主张对社会进行革命的共产主义及主张国家民族至上的法西斯主义。

激进主义是1960年及1970年间美国著名的思维模式，在流行、政治、音乐品味、教育及社会价值上，皆摒弃安定的中产阶级价值观及保守态度。激进主义主要特色包括，激进的政治理念、性解放、煽动性时尚风格及药物使用。较狭义的说，此运动亦催化1960年代的民权及环境运动精神。

The term political radicalism (or simply, in political science, radicalism) denotes political principles focused on altering social structures through revolutionary means and changing value systems in fundamental ways. Derived from the Latin radix (root), the denotation of radical has changed since its eighteenth-century coinage to comprehend the entire political spectrum—yet retains the "change at the root" connotation fundamental to revolutionary societal change. Historically, radicalism has referred exclusively to the radical left (under the single category of far-left politics) and rarely incorporating far-right politics, though these may have revolutionary elements; the prominent exception is in the United States where some consider radicalism to include both political extremes of the radical left and the radical right. In traditional labels of the spectrum of political thought, the opposite of radical on the "right" of the political spectrum is termed reactionary.