A weighted sum of means such that the weights sum to zero.
Used to answer specific questions about differences between groups or factor levels.
\text{Contrast} = c_1 \mu_1 + c_2 \mu_2 + \cdots + c_k \mu_k \quad\text{where}\quad \sum c_i = 0.$$ $c_i$ determines what comparison the contrast represents. We choose constants based off a *weight* we wish to assign to each $\mu$.