龟，泛指龟鳖目（Testudines）的爬行动物，现存14科共341种，它们的肋骨进化成特殊的骨制和软骨护盾，称为龟甲。 龟是通常可以在陆上及水中生活，亦有长时间在海中生活的海龟。龟亦是长寿的动物，自然环境中有超过百年寿命的。像很多爬行动物一般，龟是外温动物。但是由于体内新陈代谢的作用，棱皮龟的体温要高于周围环境水温。

龟鳖目下包括现存和已灭绝物种，最早的化石记录可追溯到2亿2000万年前， 甚至早于蛇、蜥蜴和鳄鱼，因此是世界上最古老的爬行动物之一。其下很多生物都活到了现在，并包括一些极度濒危物种。

Testudinata is the group of all tetrapods with a true turtle shell. It includes both modern turtles (Testudines) and many of their extinct, shelled relatives (stem-turtles). Though it was first coined as the group containing turtles by Klein in 1760, it was first defined in the modern sense by Joyce and colleagues in 2004. Testudinata does not include the primitive stem-turtle Odontochelys, which only had the bottom half of a shell.