Fold change
Fold change is a number describing how much a quantity changes going from an initial to a final value. Fold change is calculated simply as the ratio of the final value to the initial value, i.e. if the initial value is A and final value is B, the fold change is B/A.
IC50
The half maximal inhibitory concentration (IC50) is a measure of the effectiveness of a compound in inhibiting biological or biochemical function. This quantitative measure indicates how much of a particular drug or other substance (inhibitor) is needed to inhibit a given biological process by half. In other words, it is the half maximal (50%) inhibitory concentration (IC) of a substance (50% IC, or IC50).