Solidarity

sol • i • dar • i • ty |ˌsäləˈde(ə)ritē|
noun
unity or agreement of feeling or action, esp. among individuals with a common interest; mutual support within a group

At its deepest core, it is a union deeper than sympathy and empathy. Let me give you examples:

Sympathy - When someone tells you they broke a rib.
Empathy - When someone tells you they broke a rib, when you recently broke a rib too.
Solidarity - When someone tells you they broke a rib, and it's your Siamese twin.

It doesn't get closer than that.

To put one's self in another's shoes and to feel their pain, only then can we be moved with enough compassion to seek true reconciliation. This is the idea behind John 1:14, and 2 Cor 5:21

...and the two shall be one flesh ...