'Fortify' was introduced in the Time Spiral block, it is only on one card which is 'Darksteel Garrison.' It is a Future Sight card suggesting that we will see a future type of Artifact called a 'fortification.'

702.64. Fortify

• 702.64a. Fortify is an activated ability of Fortification cards. "Fortify [cost]" means "[Cost]: Attach this Fortification to target land you control. Activate this ability only any time you could cast a sorcery."
• 702.64b. For more information about Fortifications, see rule 301, "Artifacts."
• 702.64c. If a Fortification has multiple instances of fortify, any of its fortify abilities may be used.

Translation:

• 702.64a.- This is pretty much the same as Equip except it applies between artifacts and lands.
• 702.64b. - A reference point to the rules pertaining specifically to the subtype 'fortification' on Artifacts.
• 702.64c. - Fortification and 'Fortify' stack and can use any of the choices./li>