What does a promise mean?

Many people would say From a politician, nothing!

Here are some possible definitions for what a promise might mean, which do you think is most accurate?

a) a committment to do something at all costs.
b) a committment to do something unless there are unforseen difficulties.
c) a committment which risks a loss of respect if broken.
d) a statement of intent.

What should it mean if the Green Party makes a promise?