Is it ‘Buddha’ or ‘the Buddha’?
The word buddha has two uses: as a title and as a noun. As a title, it doesn’t take the article, so we should write Buddha Maitreya (or, less well, Maitreya Buddha) without any article, and in capitals. In the context of Buddhism, there is one buddha who is more prominent than any other: Buddha Gautama Shakyamuni, the historical buddha who founded the religion. He is often refered to as the Buddha (note the capital). In other contexts, we should always talk about a buddha, buddhas or the buddha(s).