There is a need for a clear definition of exacerbations used in clinical trials in patients with bronchiectasis. An expert conference was convened to develop a consensus definition of an exacerbation for use in clinical research.A systematic review of exacerbation definitions used in clinical trials from January 2000 until December 2015 and involving adults with bronchiectasis was conducted. A Delphi process followed by a round-table meeting involving bronchiectasis experts was organised to reach a consensus definition. These experts came from Europe (representing the European Multicentre Bronchiectasis Research Collaboration), North America (representing the US Bronchiectasis Research Registry/COPD Foundation), Australasia and South Africa.The definition was unanimously approved by the working group as: a person with bronchiectasis with a deterioration in three or more of the following key symptoms for at least 48 h: cough; sputum volume and/or consistency; sputum purulence; breathlessness and/or exercise tolerance; fatigue and/or malaise; haemoptysis AND a clinician determines that a change in bronchiectasis treatment is required.The working group proposes the use of this consensus-based definition for bronchiectasis exacerbation in future clinical research involving adults with bronchiectasis.