This food-specific thesaurus web-link says they are also -sometimes- known as bitters, although that would confuse at least this native speaker with the little bottles of the really sour stuff: a google search of 'bitters' brings mainly this cocktail additive up; apart from a wikipedia definition.
Bitter liqueur in contrast gives you a broad selection of famous brands of Italian amari, French digestifs, and others.

To comment on some of the above entertainment:
Again speaking as a native I can remember hearing 'digestif' in a french accent being used to describe the same by those with a penchant for sounding au fait with these strange foreign substances.
A bitter in a pub would get you a pint, a digestive in an English accent a route to the nearest corner-shop, although of course the famously hospitable and knowledgeable landlords of that green and pleasant land might guess and offer you a selection of strong alcohol.

Surely given the original as amaro it should at least contain the word bitter, after all grappa is a digestivo but not an amaro.
Bitter spirit would not apply as an amaro has always an element of sweetness (as with the word liqueuer), and a strength of around 30°, a spirit is rather completely bitter (while it can be flavoured with herbs etc.) and 40°.
HTH

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Note added at 1 day7 hrs (2008-07-30 16:36:20 GMT)
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I would not use amaro untranslated as only the cognoscenti would understand; it is not in my experience a commonly used term

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Note added at 1 day7 hrs (2008-07-30 16:39:33 GMT)
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try googling amaro on english pages and you, again, find only wiki in agreement... otherwise only Italian pages...