A pulmonary consolidation is a region of (normally compressible) lung tissue that has filled with liquid, a condition marked by induration (swelling or hardening of normally soft tissue) of a normally aerated lung. It is considered a radiologic sign. Consolidation occurs through accumulation of inflammatory cellular exudate in the alveoli and adjoining ducts. Simply, it is defined as alveolar space that contains liquid instead of gas. The fluid can be pulmonary edema, inflammatory exudate, pus, inhaled water, or blood (from bronchial tree or hemorrhage from a pulmonary artery). It must be present to diagnose pneumonia: the signs of lobar pneumonia are characteristic and clinically referred to as consolidation.
Signs that consolidation may have occurred include:
Expansion of the thorax on inspiration is reduced on the affected side
Possible medium, late, or pan-inspiratory crackles
Vocal resonance is increased. Here, the patient's voice (or whisper, as in whispered pectoriloquy) can be heard more clearly when there is consolidation, as opposed to in the healthy lung where speech sounds muffled.
Typically, an area of white lung is seen on a standard X-ray.  Consolidated tissue is more radio-opaque than normally aerated lung parenchyma, so that it is clearly demonstrable in radiography and on CT scans. Consolidation is often a middle-to-late stage feature/complication in pulmonary infections.
^Metlay, JP, Kapoor, WN, Fine, MJ (1997). "Does this patient have community-acquired pneumonia? Diagnosing pneumonia by history and physical examination". JAMA: the Journal of the American Medical Association278 (17): 1440–5. doi:10.1001/jama.278.17.1440. PMID9356004.
^Talley, Nicholas Joseph (2001). Clinical Examination, a Clinical Guide to Physical Diagnosis, Wiley, 4th ed., p. 121, ISBN 0632059710.
^Corne, Jonathan; Carroll, Mary and Delany, David (2002). Chest X-Ray Made Easy. Churchill Livingstone. ISBN0-443-07008-3.