Hi, I don’t quite understand the nature of a tick.
First, I read from several sources and it seems there are two definition:
If I combine both ideas then it would mean that a trade will always result in a change in either price, which I am not sure is true. Can someone verify if either idea is true.
Thank you
My understanding is that a tick is a change in price no matter how big or small the change. Not necessarily minimum of 1 pip but most brokers have minimum price change of 0.1 pip.
I think generally, a tick in forex represents the minimum price change (on the right side of the decimal point). 
I think it updates regardless of whether or not you enter a trade. So if the price of EUR/USD changed from 1.1716 to 1.1712, you have 4 ticks.
I think there’s also such a thing as bid tick, which could be closer to your first definition.
A tick is the least cost fluctuation. Eg: If the EUR/USD moves from 1.36543 to 1.36547 this is 4 TICK, 10 ticks in Forex equals 1 pip (Applicable to majors)
@jonandthink - Your answer # 2 is the closest I can see on this thread -
Here’s a book based on tick charts - which are smoother and more logical since a 70 tick candle may print in 30 seconds or 2 hours - his other book is based on teh same principles but relates to 5 minnute charts for those of us who do not have free access to tick charts.
I understand 1 “tick” to represent a single transaction - so 70 transactions takes as long as it takes - then prints a candlestick.
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[Edit the small increments of pips - ie 1/10th of a pip are called “pipettes” - ticks are something different - effectively I think what the old master “tape readers” - like Livermore, Wyckoff and their contemporaries used to “Feel” through their fingers !
If you go here
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and open up some of the images - you can see what a 70 pip chart looks like 
It’s the smallest change in price to the right of the decimal.