pneu·mo·nia (no?o-m?n?y?, nyo?o-)n. An acute or chronic disease marked by inflammation of the lungs, usually caused by a bacterium, virus, or other infectious agent.[New Latin, from Greek pneumoni?, lung disease, alteration (influenced by pneuma, breath) of pleumoni?, from pleum?n, lung; see pleu- in Indo-European roots.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.pneumonia (nju??m??n??) n (Pathology) inflammation of one or both lungs, in which the air sacs (alveoli) become filled with liquid, which renders them useless for breathing. It is usually caused by bacterial (esp pneumococcal) or viral infection[C17: New Latin from Greek from pneum?n lung]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014pneu?mo?nia (n??mo?n y?, -?mo? ni ?, ny?-) n. 1. inflammation of the lungs with congestion. 2. an acute infection of the lungs caused by the bacterium Streptococcus pneumoniae. [1595?1605;