Logical definition -
It is a delicate bond between two beings. The key component for its formation is likeness. As much likeness a person shares like so profound friendship becomes. Whence it requires purity and warmth in heart to built it more powerful and resistant.
Colloquial definition -
It is the most beautiful kinda patnership between two souls, which are linked with the chains of affection, trust and loyality. There Closeness gives remembrance of blood ties between the two.