There is no "one" definition, but below are a few good examples that go a little more in-depth to explain the foundations of assessment and evaluation.
Assessment and evaluation are used as ordinary language synonyms. When a narrower referent is intended, the terms are modified, as in “assessment of student learning” or “evaluation of academic services.”
Source: Higher Learning Commission. (2017). Criteria for Accreditation Terminology. Retrieved from Policies: Higher Learning Commission: https://www.hlcommission.org/Policies/glossary-new-criteria-for-accredit...
“Assessment is deciding what we want our students to learn and making sure they learn it.”
Source: Suskie, L. (2018). Assessing Student Learning: A Common Sense Guide. San Francisco: John Wiley & Sons, Inc.
“Assessment is any effort to gather, analyze, and interpret evidence which describes institutional, divisional, or agency effectiveness.”
Source: Upcraft, M. L., & Schuh, J. H. (1996). Assessment in Student Affairs: A Guide for Practitioners. San Francisco: Jossey-Bass.