What do you mean by institutions

Institutions are usually defined as certain enduring and accepted forms of procedure governing the relations between individuals and groups.
Every organisation depends upon certain recongnised and established set of rules, traditions and usages.
These usages and rules are the institutions.
They are the forms of procedures which are accepted and recognised by society and govern the relations between individuals and groups.
People form an association to satisfy their needs.
But these needs are also satisfied through institutions.
So every association has its own institutions. Without these, no association can regulate the members.
For example, family in association has its own institutions like marriage, the system of property and inheritance. A state as an association has its own institutions like government, legislative procedure, constitutions etc.