means the unity and mutual support shared by a group of people who stand together around a common cause, interest, or sense of belonging

from From the French solidarité, which grew out of the legal term solidaire — describing a debt or obligation shared jointly, where each person is liable for the whole. That traces back to the Latin solidus, 'solid, whole, complete' (the same root behind 'solid' and even the Roman gold coin, the solidus). So buried in the word is a striking idea: that a group can be bound together as completely as one unbroken thing — what's owed by one is owed by all.