When referring to the coverage of a base in a reference genome, the number of reads covering that base in an alignment. When referring to the coverage of a genome in a sequencing experiment, the average number of times that a base was sequenced. = total number of sequenced bases / genome size. For example, a 100x coverage means that each base was sequenced on average 100 times. For the human genome, that would be ~ 300 Gigabases.