# What defines a deviation vector

In the last paragraph in Wald p. 224, he states that if one considers a timelike geodesic ##\gamma## with tangent field ##\xi## and ##p \in \gamma## and now looks at the congruence of geodesics passing through ##p##, then every Jacobi field which vanishes at ##p## is a deviation field for this congruence.

Questions:

1. What properties actually defines a deviation field?
2. Why is the statement above true?