The term “maintenance of activities” is used in the Canada Labour Code, but many people speak about the “maintenance of activities” in the Canada Labour Code as meaning essential services. There may be some dispute among parties as to whether there's actually any difference between maintenance of activities and essential services.
Maintenance of activities, as expressed in the Canada Labour Code, goes to the safety and health of the public. A book on labour terms in Canada put out by Sack and called Labour Law Terms defines essential services just in relation to ensuring the safety and health of the public.
There are other people who have a different definition of essential services, which they sometimes define as involving both the safety and health of the public as well as having an economic impact. Of course, the term “essential services” is not used in the Canada Labour Code. If you look in the margin, the only term that appears is “maintenance of activities”.