The fabrication of ICs is a complex process and involves many chemical, mechanical, electrical, optical variables. Hundreds of ICs are manufactured on a single wafer at a time and the wafers are created in lots.
The integrated circuits are called die (die is singular as well as plural) when they are on a wafer and called chips when they are separated from the wafer.
The die are tested in different methods for its intended functionality, characteristics and parameters like frequency, power consumption, temperature, etc. All die are not up to the mark after manufacturing. Among the reasons to discard a die are wrong resistance, an open circuit, a missing conductor, and being physically too large or too small in some dimension.
Yield is the fraction goods of chip among the total number chips manufactured. The total number may be total chips of a single wafer or total chips of a lot. The yield analytically: