Does grease spell when used on floor cause difficult terrain or not?

[cablop]

I think the spell must be rewritten according to the answer or an errata must be added: No, this is not difficult terrain or yes, it is then movement costs double, instead of your speed is halved.

[ShieldLawrence]

As of right now, Grease is not difficult terrain and must be moved through at half speed. This is fairly clear. You go by the spell's text.

[Gwen Smith]

cablop wrote:

I think the spell must be rewritten according to the answer or an errata must be added: No, this is not difficult terrain or yes, it is then movement costs double, instead of your speed is halved.

There are other reasons that your speed is reduced outside of difficult terrain. Any abilities that let you ignore difficult terrain do not counter these other reasons.

And your speed is not "halved" in Grease. The text of the spell says, "A creature can walk within or through the area of grease at half normal speed with a DC 10 Acrobatics check. Failure means it can't move that round (and must then make a Reflex save or fall), while failure by 5 or more means it falls."

If you fail the Acrobatics check, you can't walk through it at all; if you fail by 5 or more, you fall down. None of this is true of "normal" difficult terrain: the spell is much worse.