What is diagnostic medical ultrasound?

Diagnostic medical ultrasound is an imaging procedure that uses high frequency sound waves (ultrasound) to produce sonograms (images) of organs, tissues, or blood flow inside the body. Ultrasound is like ordinary sound except it has a frequency (or pitch) higher than human beings can hear. When sent into the body from a transducer resting on a patient’s skin, the sound is reflected off internal structures. 
The returning echoes are received by the transducer and converted by an electronic instrument into an image of the internal structures on a viewing screen. These continually changing images can be recorded on film, paper, videotape, or computer.

This procedure may be called an ultrasound scan, ultrasound exam, sonogram, or an echo. 
Diagnostic ultrasound imaging is commonly called sonography or ultrasonography.

Diagnostic medical ultrasound is often related with imaging babies during pregnancy, but that is just one exam it is used for.
 Ultrasound is used to examine many parts of the body, such as the abdomen, breast,  musculoskeletal, reproductive system, heart and blood vessels. Diagnostic medical ultrasound is increasingly being used in the detection of heart and vascular disease, as well as for the detection of injuries to muscles, tendons and ligaments. 
Ultrasound does not use radiation, unlike many other imaging procedures such as x-rays or CAT scans.

Excerpt from the Medical Ultrasound Fact Sheet from the American Institute of Ultrasound in Medicine (AIUM)