The term Market Strength is a little ambiguous. It may refer to refers to the broader market rather than single stock or commodity; but it may also refer to the strength of a particular trend of a given stock or commodity. The former may refer to a market index, such as the Dow Jones Industrial Index (DJI) or the number of advancing and declining stocks in a market; while the latter takes volume or open interest into consideration.
The advance/decline line and TRIN are popular Market Strength indicators for the broader market. The popular Market Strength indicators that can be used for individual stocks and commodities include On Balance Volume (OBV), Accumulation/Distribution (A/D), the Chaikin A/D Oscillator, the Money Flow Index (MFI), and Price By Volume (PBV).