A Transformation Summon (変身召喚Henshin Shōkan) is an alternate method of Special Summoning Fusion Monsters from the Extra Deck through specific cards, without using any Fusion Materials. The prominent examples of series that use this Summon are the "Masked HERO" Archetype from the Yu-Gi-Oh! GX manga in canon and the "Morphoser" Archetype on this Wiki.
It essentially reduces the amount of cards used for a Fusion Monster by one, in that only one monster and a card such as "Mask Change" or "Form Change" is needed, as opposed to two or more monsters and "Polymerization". However, the produced Fusion Monster tends to be less powerful than the result of a traditional fusion.
Under normal circumstances, a Transformation Summon is not treated as a Fusion Summon, though some exceptions may exist. The term "Transformation Summon" only actually exists on cards in the GX manga: OCG/TCG cards and anime cards post-GX do not use it. However, the term may be used by choice of the Duelist when performing this type of Summon.
It's rumored that another variation of Transformation Summon may be coming soon; one that uses Traps as the main Summoning method and can operate similar to Pendulum Summon