You are an economist working for the Central Bank of Econville. The bank has collected quarterly data on the country's Gross Domestic Product (GDP) and inflation rate for the past 20 years. Your task is to analyze this data to understand the economic trends and provide policy recommendations. Using the Eviews software to conduct your econometric analysis, you performed an Augmented Dickey-Fuller (ADF) test on the GDP and inflation data to check for stationarity and the results show that the p-value is greater than the chosen significance level 0.05%. Assuming that both series are stationary, you estimated a simple linear regression model with GDP as the dependent variable and inflation as the independent variable. Explain the concept of stationarity in a time series context.