Repetition occurs when multiple sets of measurements are made during one

Repetition is 
The act or process or an instance of repeating or being repeated. A recitation or recital, especially of prepared or memorized material.Replication- The process of duplicating or producing an exact copy of a polynucleotide strand such as DNA.
What's the difference between them?
Repetition occurs when multiple sets of measurements are made during one scientific investigation. Replication occurs when a scientific investigation is reproduced by another person.

Repetition is multiple trials, when replication is when you redo the entire experiment. 

Stimulants speed up the central nervous system.