Is Buddhism a Religion or a Philosophy?

It is both although we would not go far wrong if we called it a practical philosophy so old that it has religious forms. We meditate before a Buddha image on an altar, bow either towards the image or towards each other and even place offerings on the altar. By contrast, we do not practise analytic philosophy before an image of Socrates on an altar. Buddhist teaching addresses philosophical issues:

the concept of souls;
causality;
the eternality or otherwise of the world;
the constituents of consciousness;
how to live rightly.

As an analysis of the cause of suffering and a way to the end of suffering, Buddhist philosophy is practical, not academic. Buddhist practice refers neither to the gods nor to a creator. The religious formulae of a temple or meditation hall facilitate the practice of meditation. They do not placate deities. So far, this makes Buddhism philosophical, not religious. In Buddhist countries, the culture incorporates both Buddhist and polytheist practices. Further, Buddhist practice does refer not to a transcendent being but definitely to a transcendent state. Since I define religion basically as response to the highest transcendence, I regard Buddhism as both philosophical and religious.

Marxism also incorporates both philosophical analysis and practical action. Both Buddhism and Marxism address the state of the world and what is to be done about it.