Issue (glossary)
(1) An area of concern that may impact the achievement of program/organizational objectives – a problem (existing), a risk (future uncertainty), or lack of information (existing). (PSM 2010)
(2) A concern that has a probability of occurrence equal to one, a consequence of occurrence greater than zero, and a time-frame in the future. An issue will occur and have a negative impact to the project, but it will not immediately occur. (Conrow 2008)
Source
(1) PSM. 2010. Practical Software and Systems Measurement: A Foundation for Objective Project Management, version 4.0. Bethesda, MD, USA: Practical Software and Systems Measurement.
(2) Conrow, E. 2008. "Risk Analysis for Space Systems." Space Systems Engineering and Risk Management Symposium, 27-29 February, 2008, Los Angeles, CA, USA.
Discussion
Definition (2) defines issue in terms of probablity of occurrence, consequence of occurrence, and time-frame; all of which are measurable.