The term "coverage" is taken in its literal meaning i.e. how much of the intended genome is covered in your sequencing. Though coverage can be expressed in terms of "breadth" or "depth", it is taken in terms of depth by default. The depth of coverage indicates how many times a nucleotide in the intended genome has got sequenced. The breadth of coverage is portion of the intended genome covered at some depth in a dataset.

Here is a link to a wiki-page roughly illustrating these terms:

http://www.metagenomics.wiki/pdf/definition/coverage-read-depth

http://www.metagenomics.wiki/pdf/definition/coverage-read-depth