Here is the definition directly from the IRS web site plus a few rules:
To qualify for the special depreciation allowance, your property must meet the following requirements.
It is new property of one of the following types.
Property depreciated under the modified accelerated cost recovery system (MACRS) with a recovery period of 20 years or less. See Can You Use MACRS To Depreciate Your Property and Which Recovery Period Applies? on pages 7 and 23, respectively, in Publication 946.
Water utility property. See 25-year property on page 22 in Publication 946.
Computer software that is not a section 197 intangible as described in Computer software on page 5 in Publication 946. (The cost of some computer software is treated as part of the cost of hardware and is depreciated under MACRS.)
Qualified leasehold improvement property (defined later).
It meets the following tests (explained later under Tests To Be Met).
Acquisition date test.
Placed in service date test.
Original use test.
It is not excepted property (explained later under Excepted Property).
Qualified leasehold improvement property. Generally, this is any improvement to an interior part of a building that is nonresidential real property, provided all of the following requirements are met.
The improvement is made under or pursuant to a lease by the lessee (or any sublessee) or the lessor of that part of the building.
That part of the building is to be occupied exclusively by the lessee (or any sublessee) of that part.
The improvement is placed in service more than 3 years after the date the building was first placed in service.
However, a qualified leasehold improvement does not include any improvement for which the expenditure is attributable to any of the following.
The enlargement of the building.
Any elevator or escalator.
Any structural component benefiting a common area.
The internal structural framework of the building.
Generally, a binding commitment to enter into a lease is treated as a lease and the parties to the commitment are treated as the lessor and lessee. However, a binding commitment between related persons is not treated as a lease.
Related persons. For this purpose, the following are related persons.
Members of an affiliated group.
The persons listed in items (1) through (9) under Related persons on page 8 of Publication 946 (except that "80% or more" should be substituted for "more than 10%" each place it appears).
An executor and a beneficiary of the same estate.
Tests To Be Met
To qualify for the special depreciation allowance, the property must meet all of the following tests.
Acquisition date test. Generally, you must have acquired the property either:
After September 10, 2001, and before September 11, 2004, but only if no written binding contract for the acquisition was in effect before September 11, 2001, or
Pursuant to a written binding contract entered into after September 10, 2001, and before September 11, 2004.
Property you manufacture, construct, or produce for your own use meets this test if you began the manufacture, construction, or production of the property after September 10, 2001, and before September 11, 2004.
Placed in service date test. Generally, the property must be placed in service for use in your trade or business or for the production of income after September 10, 2001, and before January 1, 2005.
If you sold property you placed in service after September 10, 2001, and you leased it back within 3 months after the property was originally placed in service, the property is treated as placed in service no earlier than the date it is used under the leaseback.
Original use test. The original use of the property must have begun with you after September 10, 2001. "Original use" means the first use to which the property is put, whether or not by you. Additional capital expenditures you incurred after September 10, 2001, to recondition or rebuild your property meet the original use test.
The following property does not qualify for the special depreciation allowance.
Property used by any person before September 11, 2001.
Property required to be depreciated using ADS. This includes listed property used 50% or less in a qualified business use.
Qualified New York Liberty Zone leasehold improvement property (defined next).
The link to see this for yourself is:
Please feel free to ask if you need additional assistance.