verb
-ens; -ened; -ening
to become or to cause (something) to become deep or deeper [+ obj]
to become or to cause (something, such as a feeling or emotion) to become stronger or more powerful [+ obj]
[+ obj] :to make (your knowledge, understanding, etc.) fuller or more complete
to become or to cause (something) to become worse or more severe [+ obj]
to become or to cause (something, such as a person's voice) to become lower in sound [no obj]
[no obj] :to become darker or stronger in color
To deepen your breathing is to take more air into your lungs when you breathe.