Solidarity is the responsibility one bears as a community for each one of the members, as well as the responsibility of each member for the community.
In Judaism, the sages fixed a legal and moral rule regarding solidarity – "All of Israel are responsible for one another". The original meaning was that each Jewish person is responsible for the religious observance of his fellow Jew. In modern times, the saying took on a new meaning – that each Jew is responsible for the well-being of his fellow Jew. The prominent examples of solidarity in our days are the various voluntary organizations that provide aid for citizens, free of charge.
In the twentieth century, the principle of solidarity in the country of Israel was manifested through the ideology of a co-operative; co-operative social organizations. Specifically, the Kibbutz and other similar working communities that were founded on this ideological basis.
In the book of Numbers, chapter 32, we learn about the two-and-a-half tribes: Reuben, Gad and half of Menashe, who requested permission from Moses to live outside of the borders of Israel, to the east of the Jordan River, due to the area being richly vegetated and suitable for their large flock.
Moses agreed to their request, only after they expressed their responsibility towards the rest of the Jewish people in helping them conquer their share of land in the land of Israel. They promised to first help their brethren, the remaining tribes, to conquer their land, in fighting for the said land, and only after everyone will be settled, could they return to their land. The two-and-a-half tribes agreed that they would leave their wives, children and flock on the eastern side of the Jordan River, and the men would be the ones to help the rest of the tribes to conquer the land of Israel.
The opposite of solidarity is ignoring one's obligations.