What is defined as a set of instructions written in a programming language that a computer can execute?

The definition provided in the question aligns perfectly with what a program is. A program consists of a sequence of instructions that tells the computer how to perform specific tasks. These instructions are typically written in a programming language, which can be understood and executed by the computer's processor.

In contrast, an algorithm refers to a step-by-step procedure or formula for solving a problem, which may exist independently of any programming context. A data structure is a way of organizing and storing data so that it can be accessed and modified efficiently, but does not itself represent executable instructions. A function, while part of a program, typically represents a specific, reusable piece of code designed to perform a particular task within the broader context of a program, rather than being the complete set of instructions that constitutes an entire program. Therefore, the term that best fits the definition given is indeed a program.