From WikiPedia:
Gaslighting is a form of psychological abuse in which false information is presented to the victim with the intent of making them doubt their own memory and perception.
My ex used to do stuff like come up to me and say, "So, are you ready to go do [whatever]?" And I would be saying "What are you talking about?". "Don't you remember that you agreeed to do [whatever]?" " Uh, no". "Well, its just you. You are forgetful/stupid/being obstinate". Or moving my keys and putting them back later. Rearranging stuff, and insisting nothing had changed. Stuff like that.
It is essentially a technique used to make people doubt their own conclusions. The term comes from a movie called "Gaslight"---again from WikiPedia:
The plot concerns a husband who attempts to drive his wife to insanity by manipulating small elements of their environment, and insisting that she is mistaken or misremembering when she points out these changes. The title stems from the husband's subtle dimming of the house's gas lights, which she accurately notices and which the husband insists she's imagining.
In terms of an alcoholic relationship, it is like when you call them on a promise or an agreement that you came to, and they insist that you heard them wrong, or the discussion never happened. Or if you recount an incident of abuse or neglect, they insist it wasn't as bad as you say it was, or the details are different or you are just making crap up to give them a hard time. Stuff like that.