Many people in Europe use the term 'bat' to refer to kiln shelves (i.e. UK). Thus 'bat wash' is kiln wash. However in North America, 'bat' most often refers to wood, plastic, or plaster disks which are used on the potters wheel. Bats are held in place by pins, an interlocking arrangement, a sticky membrane or glued on by moist clay or slip. Ware can then be thrown on the bat and the whole thing removed to make another piece.
Making your own plaster bats is easier than you might think
Using this rubber mold I have just made 8 - 12" bats and I still have 20 lbs of plaster left in the bag! Just just weigh 1600 grams plaster, dump it in 1120 grams of water, wait 4 minutes, mix 4 minutes and pour. As soon as the water at the top disappears, dump the next batch of plaster in the water and repeat (by the time the next plaster is ready to pour you can remove the last bat from the mold). If you are in a drier climate and make wide shapes, especially with a porcelainous clay, using a plaster bat is an excellent way to get even drying and avoid cracking. Using a BatMate you can stick them down to the wheel very hard, yet they are easy to get off. I would never use any other bat than these. You can buy one at Plainsman Clays for $75 plus shipping.