Definition of 'feature coverage'
Depends on the context it may mean differently. One common usage of the term is when not all the data points have valid/reliable values for a specific feature. For example for a feature set of $x=\{x_1, x_2, \dots, x_p, \dots, x_n\}$ not all data points may have specific feature $p$. 

It is possible that some data points lack a value for feature $p$ either because of data corruption, noise or etc. In this case the percentage of data point which actually have a reliable value for this feature is called the feature coverage of feature $p$ for this specific data set.