Tracing Your Family Tree in Ontario (at About.com):
"The counties were further sub-divided into townships... . Townships are divided into bands of land 1 1/4 miles wide called concessions. Each concession is labeled with either a number (usually a Roman numeral) or a letter (this means the concession doesn't run the length or width of the township due to the interruption of a body of water)."
A hint from Olive Tree: "Agricultural [census] returns provide information such as lot and concession number... ."
The Olive Tree blog also supplied a how-to in finding information on the agricultural census schedule(s):
1861 Canadian Census
Keyword: Agricultural for Lot/Concessions (1851 and 1861)
Census and Enumerations from the Library and Archives of Canada:
"For the 1851 and 1861 Census, the agricultural returns are listed by the name of the head-of-household.The agricultural returns for 1881, 1891, 1901 and 1911 were not retained.
1851: For each sub-district (e.g., township), the agricultural returns follow immediately after the personal returns.
1861: For each county, the agricultural returns follow immediately after the personal returns for the whole county.
1871: The personal returns for each enumeration sub-district appear in schedule one. The agricultural returns are schedules three to five and are keyed to schedule one by page and line number rather than by name."