A function(in C/C++ language) to be executed on GPU is called a Kernel. While defining kernel, a function is prefixed with keyword __global__.
__global__ void matadd(int *a, int *b)
//code to be executed on GPU
Here matadd() is a kernel.
Note: Only those kernels, which are called from cpu function(e.g. main() funtion), are prefixed with __global__ during their definition while kernels which are called from another kernel, are prefixed with __device__ during their definition.