The instructions for enumerators (page 6) under item 48 says:
In column 6 the head of each family or household will be entered as
such, and all others according to the relationship -- as wife, son,
daughter, servant, boarder, lodger, partner, etc. The persons in an
institution may be described as officer, inmate, patient, prisoner,
Not much help there. Can you find other historical records that contain both people (e.g. newspapers) that might shed more light on the relationship? Do they appear in the same household in any other census years?
Note that the support materials for the 2011 Census at Statistics Canada, includes a Census Dictionary. The entry Census Family, notes that the definition of a census family has changed over time. There are several references to 'partners' referring to common-law marriages, and notes that "As of 2006, a married couple may be of opposite or same sex."
To answer definitively we would need to find similar materials that were produced at the time of the 1901 Census, or at least records closer in time to 1901.
The instructions for the 1920 US Federal Census are more explicit:
[section 101]. Family defined.-The word "family," for census purposes, has a somewhat different
application from what it has in popular usage. It
means a group of persons living together in the same dwelling place.
The persons constituting this group may or may not be related by ties
of kinship, but if they live together forming one household they
should be considered as one family. Thus a servant who sleeps in the
house or on the premises should be included with the members of the
family for which he or she works. Again, a boarder or lodger should be
included with the members of the family with which he lodges, but a
persons who boards in one place and lodges or rooms at another should
be returned as a member of the family at the place where he lodges or
[section 111]. If two or more persons share a common abode as partners, write head for one and
partner for the other or others.
If the Canadian usage is the same, then in census-speak the "head" and "partner" could be what we call "roommates" today.