A correspondent writes: 'What, if any, is the difference between tolerance and toleration? As far as I know, tolerance is the commoner word. Are they synonymous?'
Tolerance is certainly the commoner word - try a Google search and you'll see it's about forty times as common. But that's not the issue. There are certainly meaning differences. Tolerance is the broader concept. It is used in a variety of settings, some quite specialized, especially in engineering (where people talk of tolerances, meaning a permitted deviation from a specified norm), biology (where it means the ability of an organism to withstand a particular environmental condition), and in medicine (where people talk of a tolerance for a certain drug, say). In its most general sense, it expresses a sympathetic willingness to accept views or behaviour other than one's own.
You also find tolerance quite often used as part of a compound expression, as in tolerance dose, tolerance level, and tolerance limit. A 2007 proposed addition to the OED notes tolerance zone - a designated area in which prostitution is tolerated by the local authority. That's a 20th-century usage.
Toleration is usually used when someone wants to talk about a specific instance of tolerance - a particular act of allowing something. In history, there are some famous cases of this kind, as reflected in such government names as Act of Toleration and Toleration Bill. It also has a much stronger suggestion of limits, and sometimes even a sense of reluctance. Tolerance has more positive connotations (a desire to accept) than toleration, which can mean 'we have to put up with this'. Compare the phrase religious tolerance with religious toleration. The country which practises the former is more likely to be enthusiastically supporting religious diversity than the latter.
During the history of these two words, there have certainly been occasions when their meanings have overlapped. But they seem to be pretty distinct today. Note that both have the same opposite: intolerance. There is no intoleration.