1) A molecule (as a short strand of RNA or DNA) whose presence is required for formation of another molecule (as a longer chain of DNA).

2) Short preexisting polynucleotide chain to which new deoxyribonucleotides can be added by DNA polymerase.

3) A primer is a short, single-stranded DNA sequence used in the polymerase chain reaction (PCR) technique. In the PCR method, a pair of primers is used to hybridize with the sample DNA and define the region of the DNA that will be amplified. Primers are also referred to as oligonucleotides.