Definition (1):
Preventive checks refer to Malthus’s term for activities and measures in which people attempt to prevent births in some manner.
Definition (2):
Preventive checks cause by human activities that reduce the birthrate. According to Thomas Malthus, this term largely means the marriage postponement to late ages. Positive checks are other checks on population for avoiding overpopulation. Positive checks consist of anything that works for increasing the death rate (such as famine, war, epidemics). While any of these positive checks work to decrease the population growth rate, according to Malthus, the preventive checks cannot be strong enough to stop the population from increasing faster than the means of sustenance. Whenever any drastic positive check will come, that humans will be bound for re-producing. Eventually, increased human numbers will be decreased by increased deaths.
There is also another argument, that population growth always does not only indicate more mouths for feeding but also more hands that are able to feed them. China can be a good example in this regard. So, Malthus is correct that prevention can be the best alternative. It can only take place through ongoing investment in human capital, specifically, the more often neglected half of the population, i.e. women.