What is Marginal Propensity to Consume?

What is Marginal Propensity to Consume? How is it related to Marginal Propensity to Save?

Marginal Propensity to Consume is the ratio of change in consumption expenditure ($\Delta C$) to
change in income ($\Delta Y$). So, MPC = $\Delta C$ / $\Delta Y$
Marginal Propensity to Save is the ratio of change in saving ($\Delta S$) to change in income
($\Delta Y$).
MPS = $\Delta S$ / $\Delta Y$

Aggregate of Marginal Propensity to Consume and Marginal Propensity to Save is equal to one. or
MPC + MPS =1
This is because $\Delta C$ / $\Delta Y$ + $\Delta S$ / $\Delta Y$ = $\Delta Y$ / $\Delta Y$ = 1 ($\Delta C$ + $\Delta S$ = $\Delta Y$)

So, MPC + MPS =1,
or MPC = 1 - MPS, or MPS = 1 - MPC