Weak form efficiency
One form of the Efficient Market Hypothesis (EMH).
The EMH is the general hypothesis that markets operate efficiently. In other words that assets are fairly priced by the market mechanism to incorporate available information.
There are three forms of potential efficiency: the weak form, the semi-strong form and the strong form.
The weak form states that past prices are no guide to future prices, so charting techniques cannot be used to make excess returns.