1. communication between people or groups who work with each other:

He blamed the lack of liaison between the various government departments.

There is an unfortunate lack of liaison between the departments.

The police have appointed a liaison officer to work with the local community.

2. someone who helps groups to work effectively with each other

She served as a liaison between the different groups.

3. relationship: a sexual relationship, especially between two people not married to each other

He's had a number of liaisons.