Federal, provincial and territorial laws adopt various definitions of what constitutes "archaeological interest." In general, it is safe to assume that archaeology includes:
all evidence of human occupation that (according to federal legislation) is over 75 years old (the provinces are less specific about age), and that comes out of the ground (or underwater). (The only exception is in Nova Scotia where, for provincial purposes, buried treasure is not defined as "archaeology" but is subject to similar legal rules.)
Everywhere but in Alberta, the law applies not only to such items in the ground, but on the ground (or even above the ground in Ontario and British Columbia, for old carvings in rock or trees).
Under federal law, and in most provinces and territories (explicitly in Alberta, Saskatchewan, Manitoba, Nova Scotia, Newfoundland and Labrador, and Yukon; implicitly in British Columbia and Prince Edward Island), the law also protects palaeontology (the remains of prehistoric animals and plants). (The legal status of palaeontology is less clear in Ontario, New Brunswick and the Northwest Territories; it is excluded in Quebec's law.)
Where there is any doubt as to whether a find qualifies, it is best to obtain expert advice immediately.