By looking at the articles (see links above) in the dictionary, it seems that they are synonym. Does that mean that they are interchangeable in any case or it depends on context?
Both although and even though are used to mean "despite the fact that".
However, the latter is more emphatic than the former. For example:
- Although he worked hard, he failed the test.
- Even though he worked hard, he failed the test.
The use of even in the latter sentence puts emphasis on though; it makes the contrast between the main and subordinate clauses stronger or more emphatic.
Looking up their definitions in the Longman Dictionary of Contemporary English Online, we can draw the line between them:
1. used to introduce a statement that makes your main statement seem surprising or unlikely
Although in poor health, she continued to carry out her duties.
2. used to add a statement that balances or reduces the effect of what you have just said
You can copy down my answers, although I’m not sure they’re right.
Here's the other one (also from Longman Online):
even though - used to emphasize that something is true although something else has happened or is true
Even though he’s 24 now, he’s still like a little child.
It might, on the face of it, appear that they are interchangeable and/or of similar meaning; and to a certain extent it is true, but I wouldn't say that they are synonymous. What's more, there is a certain lexical difference.