What makes a martyr a martyr? (This is a word question, not a politics question)

This is a semantic or a vocabulary question – it is not my intent to begin a discussion of terrorism or foreign policy or religion, although there is little I can do to stop it.

What the question boils down to is this: can a person call himself a martyr? If I decide to kill myself in the name of some religious belief, can I claim the word “martyr” to describe what I am doing?

Or is the word “martyr” assigned as a label by others – not the actor himself? In other words – is being called a martyr something that comes from analysis of a person’s death by others?

Who decides who is considered a martyr, and on what basis?

[Example: a highly, deeply religious person is the subject.

Situation 1: He is giving a religious speech and is assassinated while doing so. Does that make him a martyr?

Situation 2: Same person. He is walking across the street to buy a newspaper, and is run over by a bus. Does that make him a martyr?