Kalmar “pessimum” ba kalma ce da ta zama ruwan dare gama gari ba a turance, amma kalma ce da ake amfani da ita a ilmin halitta don bayyana yanayin muhalli wanda bai fi dacewa da girma ko tsira ga wata halitta ko nau’in halitta ba. Fiye da ƙari, “ƙasassun” yana nufin wurin da ci gaban kwayoyin halitta ke hana shi ta hanyar abubuwa kamar zafin jiki, zafi, haske, ko wadatar abinci. Ana iya la'akari da shi a matsayin akasin yanayin "mafi kyau", wanda shine mafi dacewa ko mafi kyawun yanayi don girma da rayuwa. yanayin da yake mafi ƙarancin buƙatu ko mafi kyau.