A tangible entity represents a physical object that exists in the real world and can be perceived by the senses. These entities, such as a person, a car, or a product, are concrete and have measurable physical properties. In database modeling, a table representing EMPLOYEE or PRODUCT would be based on a tangible entity, as each record corresponds to a specific, measurable object in the real world.

A. Logical entity: A logical entity is a representation within a data model's structure; it is a component of the design, not the physical real-world object itself.

C. Virtual entity: A virtual entity is typically a data object, like a view in a database, that is derived from other entities and does not have its own persistent physical storage.

D. Conceptual entity: A conceptual (or intangible) entity represents a non-physical concept, event, or idea, such as a bank account, a course registration, or a project, which are not physical objects.

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1. Elmasri, R., & Navathe, S. B. (2017). Fundamentals of Database Systems (7th ed.). Pearson. In Chapter 7, Section 7.2.1, the text defines an entity as a "thing" or "object" in the real world with an independent existence. It distinguishes between objects with a physical existence (e.g., a person, a car) and those with a conceptual existence (e.g., a company, a job, a university course). This directly supports the definition of a tangible entity as a physical object.

2. Coronel, C., & Morris, S. (2016). Database Systems: Design, Implementation, and Management (12th ed.). Cengage Learning. Chapter 4, "Entity Relationship (ER) Modeling," defines an entity as an object of interest to the end user. It clarifies that an entity can be tangible, such as a customer or a product, or intangible, such as a bank account or a project plan.

3. Stanford University, CS 145 Introduction to Databases Course Notes. (n.d.). Lecture 3: The Entity-Relationship Model. Retrieved from Stanford University. The course materials describe entities as real-world objects or things, distinguishing between those that are physical (e.g., students, departments) and those that are conceptual (e.g., courses, enrollments). This aligns with the distinction between tangible and conceptual entities.