a): a specific type of marriage (not to be confused with an open relationship, where cheating is, mostly, mutually consentual and often the couples involved are in common-law or civil marriages) where the partners are, legally speaking, fully married, and are neither legally separated nor divorced, and still cheat on each other. Since this is not an open relationship, it can only happen when one or both partners are ignorant of the fact that they are, or may be, cheating on each other.
If neither of the partners involved are ignorant of the cheating, they should not be deemed to be an open relationship, but should be seen to be as mutually adultrous and in a lamm but choosing to do nothing about it, which is absolutely their right, since the legal should not get involved until someone decides to file charges.
b): a neutral or pejorative term, depending on the context, for a movie, book, newspaper article or tv show where the plot revolves around a couple in a lamm marriage.
An epitomy of such a scenario would be the movie Mr. and Mrs. Smith.
person a): did you see the movie Mr. and Mrs. Smith with Brad Pitt and Angelina Jolie?
person b): yup I did, I thought it was a lamm, that's what I found so funny about it.
person a): lol, I even heard that Brad Pitt and Angelina Jolie are in a lamm in real life as well.
person b): wtf? did you just say that? I find it hard to believe. Tone it down, you are really living underwater.