2. Vital to the resolution of a crisis; decisive; as a crucial election.
3. Having the form of a cross; cross-shaped.
A crucial election is like a signpost because it shows which way the electorate is moving. The metaphor of a signpost, in fact, gives us the sense of the word crucial, "of supreme importance, critical."
Francis Bacon used the phrase instantia crucis, "crucial instance," to refer to something in an experiment that proves one of two hypotheses and disproves the other.
Bacon's phrase was based on a sense of the Latin word crux, "cross", which had come to mean "a guidepost that gives directions at a place where one road becomes two", and hence was suitable for Bacon's metaphor.
Both Robert Boyle, often called the father of modern chemistry, and Isaac Newton used the similar Latin phrase experimentum crucis, "crucial experiment".
When these phrases were translated into English, they became "crucial instance" and "crucial experiment".