Definition is a rhetorical mode that can be used in something smaller or shorter than a full paper. You can use extended definition for several paragraphs only in a paper of much greater length. You also can add to a paper a one-paragraph definition--like a brief encyclopedia definition.
The research group, headed by Dr. Lukas Sommer at the Swiss Federal Institute of Technology, has identified a heretofore unknown role for the TGF cell-to-cell signaling pathway in the pathogenesis of DiGeorge syndrome.
Zellweger syndrome is a rare, inherited metabolic disorder that affects peroxisomes, organelles found in almost all body cells. Peroxisomes are responsible for many important cell processes, including energy metabolism, which means that Zellweger syndrome can severely impact the body.