@Niyas Raphy

Yes, that's kinda obvious, My question was what exactly do they mean by leaf in domains, is it the operator, the model filed or the syntax ? 

well here is the answere from odoo code a methode that checks if a leaf is a leaf, the documentation is kinda good for for this function, 

def is_leaf(element, internal=False):
    """ Test whether an object is a valid domain term:
        - is a list or tuple
        - with 3 elements
        - second element if a valid op

        :param tuple element: a leaf in form (left, operator, right)
        :param boolean internal: allow or not the 'inselect' internal operator
            in the term. This should be always left to False.