Get Department for current user using REST API

To get the department property of the logged in user -

http://siteurl/_api/SP.UserProfiles.PeopleManager/GetMyProperties?$select=Department

To get all the properties of another user -

For Office 365/SharePoint Online:

http://siteurl/_api/SP.UserProfiles.PeopleManager/GetUserProfilePropertyFor(accountName=@v,propertyName='Department')?@v='i:0%23.f|membership|user@siteurl.onmicrosoft.com'

For SharePoint 2013 On-Premise:

http://siteurl/_api/SP.UserProfiles.PeopleManager/GetUserProfilePropertyFor(accountName=@v,propertyName='Department')?@v='domain\username'