Solidarity
Etymology
From French solidarité, from solidaire "interdependent," from Latin solidum "the whole sum," from solidus "solid, whole." In Roman law an obligation "in solidum" made each party liable for the entire debt — so solidarity carries a legal core: each of us answerable for all of us.
noun · /sol-ih-DAIR-uh-tee/
Unity and mutual support among people who share an interest or cause, especially in struggle.
What it names
A handle for standing with others as though their stake were your own — support that is active and committed rather than merely felt.
Example
The other departments walked out in solidarity, though the dispute was not theirs.
Related idea
Distinct from sympathy: sympathy is a feeling directed at another’s plight from outside it, while solidarity is a commitment that binds your fate to theirs and shows up as shared risk and action. You can feel sympathy from your armchair; solidarity costs something.