For tomatoes one definition of heirloom is any open pollinated (non hybrid) variety over 40 years old, so for a whole garden I would think all vegetables would have to fit that description. Seed catalogs use the term hierloom very loosely to include all open pollinated (non hybrid) varieties.

Most people on the toms forum refer to them as OP or hybrid. I think the 40 yrs old is kind of a myth. I've also heard 25 and 50. Many people consider a stable f6 or f7 as being OP and also considered heirloom for the purposes of selling or trading seed.