Couple of different definitions.
In general, it is the things that are considered "illegal" in regular boxing, thus it is dirty. Some define it only as the holding and hitting, others will also include any tactic than is banned. Such as purposely missing and hitting with the elbow on a hook, or hitting with the forearm to the neck on a missed punch etc. But, some people also call any clinch fighting dirty boxing, because of it's closeness, it's "down and dirty". But, as you pointed out, it is not "dirty" in things like MMA where it is allowed.
Others call Filipino boxing (panantukan) dirty boxing because it's base is western boxing with a lot of the FMA limb destructions thrown in.
Mark Hatmaker, has a good dvd set out on dirty boxing tactics, and his dvd set the complete pugilist also covers this stuff. Ron Balicki has a really good dvd set out on the Filipino Boxing as well.
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