**Core Concept**
The odds ratio (OR) is a measure of association between an exposure and an outcome in case-control and cohort studies. It represents the ratio of the odds of exposure among cases to the odds of exposure among controls.

**Why the Correct Answer is Right**
The odds ratio is a fundamental concept in epidemiology and biostatistics, used to quantify the strength and direction of the association between a risk factor and a disease. It is calculated as the ratio of the odds of the exposed group developing the disease to the odds of the unexposed group developing the disease. The odds ratio can be interpreted as the relative odds of disease among the exposed compared to the unexposed.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the odds ratio. The odds ratio is not a measure of the absolute risk or the probability of an event.
**Option B:** This option is incorrect because it is a measure of the absolute risk difference, not the odds ratio. The absolute risk difference is the difference in the risk of disease between the exposed and unexposed groups.
**Option C:** This option is incorrect because it is a measure of the relative risk, not the odds ratio. The relative risk is the ratio of the risk of disease in the exposed group to the risk of disease in the unexposed group.

**Clinical Pearl / High-Yield Fact**
When interpreting odds ratios, it's essential to consider the study design, sample size, and confounding variables. A large odds ratio does not necessarily imply a strong association, as it can be influenced by the study's design and biases.

**Correct Answer: A. Odds ratio is a measure of association between an exposure and an outcome in case-control and cohort studies.**