"ICH E3 Guideline: Structure and Content of Clinical Study Reports Questions & Answers" have the clarification on protocol deviation and important protocol deviation.

Both FDA and EMA adopted the "ICH Q & A" in year 2012.

"A protocol deviation is any change, divergence, or departure from the study design or procedures defined in the protocol. Important protocol deviations are a subset of protocol deviations that may significantly impact the completeness, accuracy, and/or reliability of the study data or that may significantly affect a subject's rights, safety, or well-being." There are also some examples listed in Q&A.

Link of the Q&A (refer to question 7): http://www.ich.org/fileadmin/Public_Web_Site/ICH_Products/Guidelines/Efficacy/E3/E3_Q_As_Step4.pdf。