An Associate of Science in Nursing (ASN) is an entry-level tertiary education nursing degree. In the United States, this type of degree is usually awarded by community colleges or similar nursing schools. Some four year colleges also offer this degree. Students awarded an Associate of Science in Nursing are qualified to sit for the NCLEX-RN and apply for licensure as a Registered Nurse.
Some hospital-based nursing schools that granted diplomas altered their curriculum to offer associate degrees.
Many nursing schools offering a bachelor's degree have "fast-track" programs to enable practicing nurses with an associate's degree to earn a bachelor's degree in approximately two years. In some institutions this is called "Bridging."(wikipedia)