Dieter: Stable could be a useful synonym for thick. In fact, I have heard it used consistently by a Japanese ex-insei who very probably had never read English Go literature. He thus translated atsumi as stability.
To interpret the above: adding a stone to a dead group is a waste. So is adding a stone to a living group. Adding a stone where the opponent is strong is not useful. So is adding a stone to a position where you are strong. Play away from thickness or as the insei said: don't play close to stable positions.
RobertJasiek: Stable is a concept with a meaning of its own. "Strong" is a possible synonym for thick, provided the context excludes other uses for strong. One can also formulate a well-known principle: "Play away from strong groups." It is no good idea to apply every imprecise translation.
exswoo Would I be correct in assuming that thickness would actually be a particular case of a group of stones being stable? For example, I don't think a stable black group that is surrounded by white stones would be considered thick...
...or do you want to reword the definition of stable slightly so that it encompasses this? ;)
RobertJasiek: Thickness is (an example for being) stable. A strong, but surrounded group is passive thickness if protecting territory or similar to thickness if lacking any territory potential.