Benchmark

A yardstick — usually an index — that you compare your return with over the same days.

A benchmark answers "compared with what?". For a portfolio of large US companies it is often the S&P 500; for one full of technology companies, perhaps the Nasdaq-100. You compare your time-weighted return with the benchmark's return over exactly the same period.

The gap is stated in percentage points: 8% against 11% is 3 percentage points behind. A short period says little; luck dominates over weeks.