What do we call a function that executes specifically on the graphics processing unit (GPU)?

The term "kernel" refers to a function that is executed on the graphics processing unit (GPU). In the context of GPU computing, particularly with frameworks like CUDA, a kernel defines the set of instructions that are executed by many threads in parallel on the GPU. This design allows for efficient handling of tasks that can benefit from parallel processing, such as rendering graphics or performing large scale computations.

In contrast to other options, a kernel is explicitly designed to run in a parallel fashion on the GPU, harnessing its architecture to manage multiple threads executing the same operation concurrently. Other terms like "thread" refer to the individual execution contexts within a kernel, while "function call" and "subroutine" are more generic programming concepts and do not specifically pertain to GPU execution. The specificity of the term "kernel" makes it the correct answer in the given context.