# $I(g)$-terminology

During my studies I came across the $$I(g)$$-symbol, in the sense that $$Z_t \sim I(g)$$ for an integer $$g$$ and $$Z_t$$ is a times series. What exactly does this mean, and does it have to do with stationarity?

It's hard to tell without more context. But mostly sure it means "$$Z_t$$ is integrated of order $$g$$", i.e. $$\Delta ^g Z_t=(1-d)^gZ_t$$ (where $$d$$ is the lag operator) is stationary, in other words you have to difference $$Z_t$$ $$g$$ times to get an stationary time series. See this.