http://en.wikipedia.org/wiki/Electronic_signature#Enforceability_of_electronic_signatures
...
In the United States, the definition of what qualifies as an electronic signature is wide and is set out in the Uniform Electronic Transactions Act ("UETA"😉 released by the National Conference of Commissioners on Uniform State Laws (NCCUSL) in 1999.[9] It was influenced by ABA committee white papers and the uniform law promulgated by NCCUSL. Under UETA, the term means "an electronic sound, symbol, or process, attached to or logically associated with a record and executed or adopted by a person with the intent to sign the record." This definition and many other core concepts of UETA are echoed in the U.S. ESign Act of 2000.[1] 47 US states, the District of Columbia, and the US Virgin Islands have enacted UETA.[10] Only Illinois, New York (New York recognizes Electronic Signatures and Records Act (ESRA)[11] and Washington State have not enacted UETA.[12]
...
Legal definitions
Various laws have been passed internationally to facilitate commerce by the use of electronic records and signatures in interstate and foreign commerce. The intent is to ensure the validity and legal effect of contracts entered into electronically. For instance,
...
ESIGN Act Sec 106 definitions
[14]
(2) ELECTRONIC- The term 'electronic' means relating to technology having electrical, digital, magnetic, wireless, optical, electromagnetic, or similar capabilities.
(4) ELECTRONIC RECORD- The term 'electronic record' means a contract or other record created, generated, sent, communicated, received, or stored by electronic means.
(5) ELECTRONIC SIGNATURE- The term 'electronic signature' means an electronic sound, symbol, or process, attached to or logically associated with a contract or other record and executed or adopted by a person with the intent to sign the record.
...
UETA Sec 2 definitions
(5) "Electronic" means relating to technology having electrical, digital, magnetic, wireless, optical, electromagnetic, or similar capabilities.
(6) "Electronic agent" means a computer program or an electronic or other automated means used independently to initiate an action or respond to electronic records or performances in whole or in part, without review or action by an individual.
(7) "Electronic record" means a record created, generated, sent, communicated, received, or stored by electronic means.
(8) "Electronic signature" means an electronic sound, symbol, or process attached to or logically associated with a record and executed or adopted by a person with the intent to sign the record.
...