A namespace is the topmost layer of an individual ontology, it will contain all classes and properties defined by it. In order to define a namespace you can do:

# These prefixes will be used in the definition of the ontology,
# thus must be explicitly defined
@prefix nrl: <http://www.semanticdesktop.org/ontologies/2007/08/15/nrl#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix tracker: <http://www.tracker-project.org/ontologies/tracker#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix nao: <http://www.semanticdesktop.org/ontologies/2007/08/15/nao#> .

# This is our example namespace
@prefix ex: <http://example.org/#>

ex: a tracker:Namespace, tracker:Ontology
    tracker:prefix "ex"
    rdfs:comment "example ontology"
    nao:lastModified "2017-01-01T15:00:00Z"