The performance of routing protocols in a mobile ad hoc network (MANET) is related to the network topology and nodes mobility. In order to ensure good performances of routing protocols in MANET, it is necessary to take into account the mobility criterion when choosing best routes in the network. In this paper, we propose a new measure of the network mobility which is relative and lightweight. Relative because it is directly dependent on neighboring and link state changes, and lightweight because it does not consume enough resources in memory and CPU. Each node estimates its relative mobility, based on changes of the links in its neighboring. This measure of mobility has no unit, it is independent of any existing mobility models and it is calculated at regular time intervals.