what is monotonicity and strict monotonicity in preferences?
The word monotonic means &quot;always moving in the same direction&quot;, in our case, always going up.
Monotonic preferences mean that the customer always prefers more of a good.
This comes in two flavors:

Strictly monotonic: More of one good is always preferred to less of that good.
Weakly monotonic: More of one good is always equivalent or better than less of that good.