COLD (chronic obstructive lung disease): Any disorder that persistently obstructs bronchial airflow. COLD mainly involves two related diseases -- chronic bronchitis and emphysema.
Both bronchitis and emphysema cause chronic obstruction of air flowing through the airways of the lungs. The obstruction is generally permanent and progresses (becomes worse) over time.
By contrast, in asthma there is also obstruction of airflow out of the lungs, but the obstruction is usually reversible and between attacks of asthma the flow of air through the airways is usually good.
COLD is also called chronic obstructive pulmonary disease (COPD).