Hot Pack (90s): A randomly inserted pack containing all inserts and no base cards. In the mid-90s, most Fleer sets came with one insert in a pack. But for 1995 Fleer and both series of 1995 Ultra, randomly inserted "Hot Packs" would yield nothing but inserts. About half the cards in a typical Hot Pack would be of the pack-specific insert with various other inserts making up the remainder. For example, a 1995 Fleer Baseball Hobby edition Hot Pack would contain six of the Hobby-only "Team Leader" inserts, with the other six being cards from the other various insert sets.
Fleer was not the only company in the mid-90s to use Hot Packs. 100 packs containing a full set of all 12 serial-numbered Donruss Elite inserts (all with matching serial numbers) were randomly seeded into boxes of 1995 Donruss Series Two.
Hot Pack (present day): An unopened pack of cards offered for sale, usually on eBay or other similar websites, that supposedly guarantees either an autograph or game-used insert.
Some products, like 2007 Ultra SE had a "hit" in every pack. However, most hot packs that are offered for sale over the internet are packs that have likely been examined by a pack searcher, and usually purchased at a mass-market retail outlet.
Of course, since there is no way of determining if an unopened pack has either an autograph or game used card until it is actually opened, it is not advisable to purchase such packs.