Self-Proving Affidavit
A self-proving affidavit is document which states that each subscribing witness was present when the testator executed the will and that, to the best of each witness's knowledge, the testator was:
1. At least eighteen years old
2. Possessed a sound mind, and
3. Signed the will freely.

The self-proving affidavit must be signed before an officer authorized to administer oaths, generally a notary public, when the will is executed. 

Generally, two people must "prove" each will that is offered for probate by appearing before the Register of Wills and signing an affidavit stating that they are familiar with the testator's signature and it is the same as the signature on that will.  A self-proving affidavit is intended to avoid this requirement. The Register of Wills may use a self-proving affidavit as proof of the testator's signature, rather than requiring two people to come in and sign another affidavit.

See: Witnesses, Advantages; Witnesses, Generally; Witnesses, Eligibility