Definition of branch?

[Tim Hill]

Sorry to spam with such a simple question ...

The User's Guide says: "A branch is an ordered set of revisions" and  
"Branches may split apart and be merged back together". From this, I  
take the definition of a branch to be a linear timeline of revisions  
that may or may not have merges to/from other branches which also have  
their own linear timeline.

However, it then also says: "A branch might have other lines of  
development..." which makes a branch sound more like an entire tree.

Trying to reconcile these two statements.

TIA,
Tim