Other forms: solidarities
When you see a group of citizens protesting something on the news, marching in a group, holding signs, and chanting slogans, you know they are in solidarity with each other, or united behind a common goal or purpose.
Any time you express support of a group or the people in it, you're showing solidarity with them. The word is used most often to describe a sense of unity with a political group, a group of striking workers, or people who have been deprived of their rights in some way. You might show your own solidarity by signing a petition, joining in a protest, or simply flashing a thumbs up.