1. The number of deaths per given unit of population over a given period of time. (wiktionary.org)
2. A measure of the number of deaths (in general, or due to a specific cause) in a population. Scaled to the size of that population, per unit of time. Mortality rate is typically expressed in units of deaths per 1000 individuals per year; thus, a mortality rate of 9.5 (out of 1000) in a population of 1,000 would mean 9.5 deaths per year in that entire population, or 0.95% out of the total. (wikipedia.org)
3. The proportion of deaths in a population or to a specific number of the population. The ratio of deaths in an area to the population of that area; expressed per 1000 per year. (biology-online.org)