The 20:20 ratio compares the average income of the top 20% richest of a given society to the poorest 20% of that society.
The Malthusian Trap states that the population increases exponentially, while the production of food only grows arithmetically.
Inequality refers to the unfair situation in a society when some people have more opportunities, money, etc. than other people.
Indifference curve is a set of combinations of two goods that gave the buyer or the customer equal satisfaction or utility.