to fluctuate / a fluctuation
If something fluctuates, it means it goes up and down. In other words, there’s no clear trend – it doesn’t rise or fall steadily. It’s common in periods of instability. For instance, petrol prices tend to fluctuate because there are so many different factors which influence them, especially when the world situation is unstable. Remember that fluctuation means movements around a point, so it’s common to say that figures fluctuate between x and y or around X. You can also say whether the fluctuation is wide, normal or slight.
My weight tends to fluctuate between 80 and 85 kg depending on the time of year, what I’m doing etc.
The value of the shares will depend on normal fluctuations in the markets, so you will need to keep an eye on any significant trends.