What does the term "mortality" refer to?

Natality and immigration contribute to an increase in population density and mortality and emigration to a decrease. (i) Natality refers to the number of births during a given period in the population that are added to the initial density. (ii) Mortality is the number of deaths in the population during a given period. (iii) Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration. (iv)Emigration is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration.