Background: Intelligibility and comprehensibility in speech disorders can be assessed both perceptually and instrumentally, but a lack of consensus exists regarding the terminology and related speech measures in both the clinical and scientific fields. Aims: To draw up a more consensual definition of intelligibility and comprehensibility and to define which assessment methods relate to both concepts, as part of their definition. Methods & Procedures: A three-round modified Delphi consensus study was carried out among clinicians, researchers and lecturers engaged in activities in speech disorders. Outcomes & Results: Forty international experts from different fields (mainly clinicians, linguists and computer scientists) participated in the elaboration of a comprehensive definition of intelligibility and comprehensibility and their assessment. While both concepts are linked and contribute to functional human communication, they relate to two different reconstruction levels of the transmitted speech material. Intelligibility refers to the acoustic–phonetic decoding of the utterance, while comprehensibility relates to the reconstruction of the meaning of the message. Consequently, the perceptual assessment of intelligibility requires the use of unpredictable speech material (pseudo-words, minimal word pairs, unpredictable sentences), whereas comprehensibility assessment is meaning and context related and entails more functional speech stimuli and tasks. Conclusion & Implications: This consensus study provides the scientific and clinical communities with a better understanding of intelligibility and comprehensibility. A comprehensive definition was drafted, including specifications regarding the tasks that best fit their assessment. The outcome has implications for both clinical practice and scientific research, as the disambiguation improves communication between professionals and thereby increases the efficiency of patient assessment and care and benefits the progress of research as well as research translation.