I’m explaining this concept – as I understand it – here because I realized people may not be familiar with it.

Syncretism is basically translating/melding/relating different religious/mythological traditions together. Don’t worry, I’ll explain what this means.

Translation mostly manifests in the form of saying that two gods from two different religions are actually the same god. You see this most often with Greek Mythology, where the Romans conflated their own Roman gods with the Greek gods, until you couldn’t tell the difference anymore.
So, for example: Jupiter and Zeus are the same guy.
This doesn’t just need to happen with gods, but it happens most often with them.
Christianity also does this, where early Christians tended to ‘translate’ their one true god to…Jupiter. Or, reportedly, the Virgin Mary with Venus.^A)

Melding is another form of syncretism, whereby you basically merge two traditions together until you can’t tell them apart anymore. See above Roman example.
Though in practice this is actually a bit more interesting.
Christians, for example, adopted a lot of Greek and Roman philosophy directly into the religion…to the point that modern Catholic doctrine can still be traced back to Greek philosophers like Aristotle and Plato.
For a more exciting example, look at Mexico and their traditions related to the dead. They’ve basically incorporated a bunch of folk traditions into Catholic Christianity, creating something that looks kinda weird to other Christians.

I’m creating a dedicated section because I feel that what I’m about to write veers away from the simple ‘what is it’ question.

Similar to melding, one thing that often happens is a one-way transfer of ideas. From what I’ve seen, this mostly happens with polytheistic religions which interact with Christianity. The Norse, for example, started adopting Christian symbolism before they underwent Christianization.
This is one-way usually because the other tradition doesn’t tend to get anything back.

Another thing to consider is ‘folklorization’ or ‘existing without conflicting’ which is when one tradition becomes dominant, but the other tradition survives in the form of folklore…or just exists alongside another.
You see this most clearly in Arthurian Legend: there are just as many Christian elements as there are pagan elements. Though another example of such would be any European folklore you’ve heard. The Wild Hunt in Western Europe is probably a remnant of Norse Mythology as scary folklore. Or Banshees and Leprechauns in Ireland.

Related to melding, but distinct is assimilation…whereby one tradition consciously assimilates another for the sake of spreading the main tradition. This happens most with Christianity and Buddhism.
Christianity would take local traditions and find a way to make them Christian. A possible example of this being Saint Brigid, who may or may not have just been an assimilation of the Celtic Goddess also named Brigid.
Buddhism is actually pretty well-known for attaching itself to local traditions. It exists alongside traditional beliefs in China…and it attached itself to Shinto to such a degree that there are people who believe Shinto didn’t exist before Buddhism.

I’d consider all of this to broadly fall under ‘syncretism’.