Thank you all for your considered replies.
I am still not sure if it is possible to produce a clear definition, the excellent examples given are, I think, judgements by responsible people, ie people who feel they have a responsibility for the safety of others.
The fall down will be where someone does not believe that responsibility lies with them (I have heard it described as 'slopey shoulder syndrome') and in the cold light of an investigation it turns out that actually they are responsible.
Some replies on here from operators presumably do give me pause for thought wrt to Duty Holder as described by Chris Kebab, Squidlord et al. Do you think it unreasonable that someone should not be responsible for ensuring that an aircraft is airworthy/fit to fly/safe? I deliberately use those terms because I think they will mean different things to different people.
If it is reasonable for that responsibility to be imposed, where do you think it lies?