I've never encountered that term. It sounds like the client's in-house shorthand for horizontal-only motion of some kind. Personally, I wouldn't be afraid to ask for clarification.
And you can get it by saying something like, "I know you want this graphic to slide horizontally from completely off the screen and fully onto the screen, but I'm not sure if you want it to come from the right or left, and I'm not sure how fast this should happen."
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA
Like Dave and you I never heard the term before- it may just be an animated graphic element that moves the shot off the screen while bringing in another (horizontal wipe like Dave mentioned). I would also ask without being embarrassed that I do not know the term and I've been doing this for over 17 years now.
With two veterans of television telling you they haven't heard of it, I'm going to say you can go ahead and ask.
My (complete and total) guess would be that it would be a graphic that comes in along the entire side of the screen and the main video slides a bit in the same direction so that whatever was centered in the shot remains centered in the newly narrower video area.
They do this on the SD broadcast of The Colbert Report quite often.
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In my broadcast station days, we called this an "animated snipe". It's really an offshoot of the "bug", which, before these things got animated, was just the station, or show, logo in the lower third area, usually on the right, so as not to conflict with network graphics or other graphical elements. I'm sure you'll find as many different names as there are stations. Many of the terms come from the printing industry, from looong before there was television, bug and snipe in particular.