IPC stands for instructions per cycle/clock. This tells you how many things a CPU can do in one cycle.
While clock speed tells you how many cycles a CPU can complete in a second, IPC tells you how many tasks a CPU can conduct in each cycle. For example, while a CPU with a faster clock speed can complete more cycles in one second, a CPU with a higher IPC but lower clock speed might still be able to complete more tasks in one second. Ultimately, what makes for a faster CPU is a mixture of both clock speed, IPC, and the number of cores.
Note that a CPU's IPC can vary based upon the workload. Further, CPU manufacturers do not typically share IPC information in spec sheets. Tom's Hardware conducts testing to determine IPC in our CPU reviews.
This article is part of the Tom's Hardware Glossary.