"Closing Tick" is the closing price of a given financial instrument at the specified time period you're using in your chart. If you're using a daily bars chart, the Closing Tick will reflect the closing price at days end. If you're using a 5 minute candle stick chart, the Closing Tick will reflect the price at the end of that 5 minute period.
Positive closing ticks indicate to traders in stock trading, options trading, and futures trading, that the stock had strength. The opposite is true of a negative Closing Tick.
Related terms: Arms Index (TRIN), Downtick, Uptick