MACD
Moving Average Convergence Divergence: the gap between two exponential moving averages, plotted with a signal line.
In plain English
The common default uses 12- and 26-period averages with a 9-period signal line. Because it is built from moving averages, it inherits their lag.
Example
The MACD line crossing above its signal line is read by some as momentum turning up — after the move has already begun.