1. Walther, L. M., & Skousen, C. J. (2009). Principles of Accounting. Utah State University. Chapter 10, "Property, Plant, and Equipment," p. 10. The text defines depreciation as "the systematic allocation of the cost of a plant asset to expense over its useful life." It further explains amortization for intangible assets in Chapter 11.
2. Franklin, M., Graybeal, P., & Cooper, D. (2020). Principles of Accounting, Volume 1: Financial Accounting. OpenStax, Rice University. Section 10.2, "Describe and Apply Depreciation Methods," defines depreciation as "the process of allocating the cost of a tangible asset over its useful life." Section 10.5 defines amortization for intangible assets.
3. MIT OpenCourseWare. (2004). 15.511 Financial Accounting, Fall 2004. Lecture Notes, Lecture 10: Long-Lived Assets and Depreciation. Massachusetts Institute of Technology. p. 2. Defines depreciation as the "process of allocating the cost of a long-lived asset to expense in the accounting periods in which the asset is used."