Maximum Deviation
The maximum deviation is a feature in the order windows of the MT4 and MT5 platforms where you can actually set a pip-range that you are prepared your fill-price to deviate. If you leave the box unchecked your order will only be filled at the price you have specified in your limit order.
If, on the other hand, you check the box and select an acceptable pip-range, your order will be filled at your limit order and at any available price within the range that you have specified. This is an especially useful tool for helping your limit order to be hit at times of high market volatility.