Contact Fusion (コンタクト融ゆう合ごう Kontakuto Yūgō) is a semi-official term which means to Special Summon a Fusion Monster with its own Summoning procedure that states "You do not use "Polymerization"
Contact Fusion is not a Fusion Summon, as it is a type of built-in Special Summon. Attempting to Special Summon via Contact Fusion opens the Summon negation window and does not start a Chain.
All monsters involved in the Special Summon must be on the side of the field of the player conducting the Contact Fusion (not the case for "Chimeratech Fortress Dragon"), and cannot be replaced by Fusion Substitute Monsters.
If a monster is face-down and you wish to use it for a Contact Fusion that shuffles the Fusion Materials into the Deck, you must reveal it before shuffling it into the Deck to allow your opponent to verify the card's identity.
This is redundant for Contact Fusions conducted via banishing or sending to the Graveyard, as their Fusion Materials can be publicly verified afterwards.
Much like all Extra Deck monsters, Fusion Monsters that use Contact Fusion can be Special Summoned from the Graveyard or the so-called Banished Zone, as long as they were successfully Special Summoned from the Extra Deck via Contact Fusion first. (Like all Extra Deck monsters, this does not apply if the monster is returned to the Extra Deck; in that case, the monster must be Special Summoned via Contact Fusion again.)
The first instances of this Summon were in the original Yu-Gi-Oh! manga, where specific monsters could be fused into a Fusion Monster without the use of "Polymerization". The first monster that was Summoned in this manner was "Valkyrion the Magna Warrior". Unlike Fusion by "Polymerization", monsters created by this method can attack in the same turn they were created.
This term was first widely introduced in Power of the Duelist, and was originally a term from the Yu-Gi-Oh! GX anime used to describe the Special Summoning method of the "Neos" Fusion Monsters, which are Summoned by shuffling their Fusion Material Monsters from the field into the Deck. This term was later applied to the "Gladiator Beast" Fusion Monsters by players, as those monsters Special Summon themselves via the same method.
The term has never appeared in the rulebook or any card effect text, but it does appear in the flavor text of "Elemental HERO Neos" and is often used in rulings.
This Special Summon can somewhat be considered to be a precursor to Synchro Summoning, as it only requires the proper monsters on the field and the appropriate monster in the Extra Deck, without the support of a Spell Card (as used in most Ritual Summons and Fusion Summons).