Here's the definition from the BATF web site:
WHAT ARE CURIOS OR RELICS?
As set out in the regulations (27 CFR 178.11), curios or relics include firearms which are of special interest to collectors by reason of some quality other than is associated with firearms intended for sporting use or as offensive or defensive weapons. To be recognized as curios or relics, firearms must fall within one of the following categories:
1.) Firearms which were manufactured at least 50 years prior to the current date, but not including replicas thereof;
2.) Firearms which are certified by the curator of a municipal, State, or Federal museum which exhibits firearms to be curios or relics of museum interest; and
3.) Any other firearms which derive a substantial part of their monetary value from the fact that they are novel, rare, bizarre, or because of their association with some historical figure, period, or event. Proof of qualification of a particular firearm under this category may be established by evidence of present value and evidence that like firearms are not available except as collector's items, or that the value of like firearms available in ordinary commercial channels is substantially less.
Here's a link to the current C&R list, again on the BATF website:
http://www.atf.gov/firearms/curios/sec2.htm-Lee
