鲸是一种分布于广大海洋的品种以及多样的完全水生海洋哺乳动物。鲸是海洋哺乳动物中鲸目生物的俗名，又分为齿鲸类（Odontoceti）和须鲸类(Mystacoceti)。鲸目包括海豚科。在日常语言中，常将鲸和海豚（以及江豚）分开，但在动物学中它们同属于一个系群。海豚科属于鲸目下的齿鲸亚目，该亚目中有抹香鲸及等。而以鲸须替代牙齿，利用鲸须过滤水中浮游生物进食的须鲸亚目，有座头鲸及现存地球上最大的动物蓝鲸等。

现存的鲸家族包括新须鲸科（Cetotheriidae，现存的唯一成员是小露脊鲸），Balaenopteridae（须鲸家族），Balaenidae（露脊鲸家族），Eschrichtiidae（灰鲸家族），一角鲸科（Monodontidae，白鲸和独角鲸），Physeteridae（抹香鲸家族），Kogiidae（侏儒鲸和小抹香鲸），Ziphiidae（喙鲸家族）。现存共有40种鲸鱼种类。两种鲸的亚目，齿鲸和须鲸，被认为在3400万年前就分化了。鲸，海豚和江豚，同属鲸偶蹄目，和它们两趾的有蹄动物血缘最近的近亲是河马，大约在4000万年前分化。

Whale is the common name for a widely distributed and diverse group of fully aquatic placental marine mammals. They are an informal grouping within the infraorder Cetacea, usually excluding dolphins and porpoises. Whales, dolphins and porpoises belong to the order Cetartiodactyla with even-toed ungulates and their closest living relatives are the hippopotamuses, having diverged about 40 million years ago. The two parvorders of whales, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have split apart around 34 million years ago. The whales comprise eight extant families: Balaenopteridae (the rorquals), Balaenidae (right whales), Cetotheriidae (the pygmy right whale), Eschrichtiidae (the gray whale), Monodontidae (belugas and narwhals), Physeteridae (the sperm whale), Kogiidae (the dwarf and pygmy sperm whale), and Ziphiidae (the beaked whales).