Leopards Ate My Face

There are various definitions of synonymity with varying degrees of strictness. Whether something is considered synonymous depends both on how strictly one defines synonymity and on which context one operates in.

I assumed a relatively strict definition: Two terms are synonymous if and only if they can be used interchangeably in most contexts, e.g. "bigger" and "larger". Under that definition, "tax" and "tariff" are not synonymous; "tariff" usually implies something crossing a border while "tax" doesn't.

However, an equally correct definition is that two terms are synonymous if they have similar or related meanings within a context. Under this definition, "tax" and "tariff" are synonymous since they describe similar things – even if they aren't interchangeable. This definition is usually used by synonym lists because it makes it a lot easier to write those lists. Annoyingly, this means that two words that are listed as synonymous in such a list aren't necessarily synonymous in the context you're using them in.

For example, Collins lists "tariff" and "tithe" as synonymous. Do you know anyone who pays a tariff to a church? The synonym list for "tithe" doesn't even mention a church-specific reading; it just assumes that a tithe is some kind of tax and that's close enough. You can write like that but your style would be seen as very flowery and wouldn't be suitable e.g. in a scientific context.

Another correct definition, by the way, is that the two words must have exactly the same meaning in all possible readings. That one is so strict it's practically useless for natural languages but can be use in different contexts.

Let's look at how Merriam-Webster describes synonyms:

1: one of two or more words or expressions of the same language that have the same or nearly the same meaning in some or all senses

2a: a word or phrase that by association is held to embody something (such as a concept or quality)
"a tyrant whose name has become a synonym for oppression"

2b: metonym

3: one of two or more scientific names used to designate the same taxonomic group
→ compare homonym

All three definitions I gave above match Merriam-Webster's first definition, depending on whether one chooses "the same" vs. "nearly the same" and "some" vs. "all".

Interestingly, Collins's definition of "synonym" is very strict due to excessive brevity:

A synonym is a word or expression which means the same as another word or expression.

This doesn't allow for similar meanings (which their own synonym lists heavily rely upon as illustrated above), which is probably not intended.

I didn't check Thesauris since you messed up that link but so far one dictionary says "it depends" and the other one says "the meaning must be the same" (and then completely ignores its own definition). "It depends" is the best we can do.