Is Buddhism a religion or a philosophy? A claim which is often heard in connection with Buddhism is that it is not a religion. From a scientific point of view, is it a religion, or is it not?
It really depends which definition of religion you go for. For instance

Human recognition of superhuman controlling power and particularly of
a personal God [..]
Oxford English Dictionary.

Buddhism is not a religion based on this definition

Any specific system belief or worship, often involving a code of
ethics and a philosophy
Websters New World Dictionary

Buddhism is a religion based on this definition
(Both quotes are taken from Jared Diamond's the world until yesterday pp 327)
Buddhism certainly is in a grey area with definitions. But then so are other philosophies/religions. Confucianism was considered a religion a few decades ago whereas now it is definitely seen as a philosophy (Jared Diamond again). Shintoism is simliar in the grey area. Going further back then Epicurianism, Stoicism and Neoplatonism all have something about the religious to them but are always (in my reading) spoken about as philosophies.
I would contend that the philosophy/religion divide is a continuum and owes something to the prevailing cultural climate. Nobody would say Marxism is a religion but there is certainly a religious attitude (controversial final point!)
Footnote
Well not quite the final point. Again Jared Diamond - he has 16 definitions for religion on pp 327 of World until Yesterday. It's an interesting exercise to go through them all and see if Buddhism fit's into each definition. It varies quite a bit. For instance just to give one more
Émile_Durkheim

A religion is a unified system of beliefs and practices relative to
sacred things, i.e., things set apart and forbidden--beliefs and
practices which unite in one single moral community called a Church,
all those who adhere to them.

Well maybe, maybe not.