Iseli (elisuka ku-Latin cella, okusho ukuthi "igumbi elincane") liyisisekelo esiyisisekelo sokusebenza, ukusebenza, kanye nokuphilayo kwazo zonke izinto eziphilayo ezaziwayo. Amaseli ayizingxenye ezincane kakhulu zempilo, yingakho kuvame ukubizwa ngokuthi "izakhiwo zokwakha impilo". Ukufundwa kwamaseli kubizwa ngokuthi yi-cell biology, biology yeselula, noma i-cytology. Amaseli aqukethe i-cytoplasm evalelwe ngaphakathi kolwelwesi, equkethe ama-biomolecule amaningi njengamaprotheni kanye nama-nucleic acid. Iningi lamaseli ezitshalo nezilwane abonakala kuphela ngaphansi kwesibonakhulu esikhanyayo, ngobukhulu obuphakathi kwe-1 ne-100 micrometres. isakhiwo seseli esiningiliziwe. Izinto eziphilayo zingahlukaniswa njenge-unicellular (equkethe iseli elilodwa njengama-bacteria) noma ama-multicellular (kufaka phakathi izitshalo nezilwane) .Izinto eziningi ezinama-unicellular zihlukaniswa njengezincane. Inani lamaseli ezitshalweni nasezilwaneni liyahlukahluka kuye ngezinhlobo zezilwane; kulinganiselwa ukuthi abantu baqukethe amaseli angaba yizigidigidi ezingama-40 (4 × 1013). Ingqondo yomuntu ibalelwa kuma-80 billion alawa maseli. Amaseli atholwa nguRobert Hooke ngo-1665, owaqamba ngokufana kwawo namaseli ahlala izindela ezingamaKrestu esigodlweni sezindela. Ithiyori yeseli, eyaqala ukwakhiwa ngo-1839 nguMatthias Jakob Schleiden noTheodor Schwann, ithi zonke izinto eziphilayo zakhiwa yinye noma amaseli amaningi, ukuthi amaseli ayingxenye eyisisekelo yokwakheka nokusebenza kwazo zonke izinto eziphilayo, nokuthi wonke amaseli avela ku amaseli akhona.