It depends very much on what precedes this phrase; "absorption" is a possibility,e.g.

"For medical applications, the frequency of ultrasound tends to lie within the range of 2-20 million cycles per second (MHz), and echocardiography sits at the lower end of this spectrum (2.5-10 MHz). During transmission through tissues, energy is lost due to absorption and scattering, and thus the reflected energy of the echo is invariably lower than the original ultrasound."


ref. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=11...