A tick size is the smallest increment in price that an equity, future, or other exchange-traded instrument is permitted to move. Tick sizes can be fixed (e.g. €0.01), or they may vary according to the current price. Larger increments are issued at higher prices, while heavily-traded equities are given smaller tick sizes. Tick size affects market quality because it limits the prices that traders can quote, and thus restricts price competition.
In Europe, the Markets in Financial Instruments Directive (MiFID) has created a variety of Multilateral Trading Facilities (MTFs) that abide by different tick-size regimens for the same equities. To account for these discrepancies, order routing systems must track every MTF's tick-size regimen, and adjust outgoing orders accordingly.
For NYSE Euronext tick sizes, please view the Organization of Trading section and click on Appendix to the Euronext Cash Market Trading Manual.