The Constitution does not define the phrase natural-born citizen, and various opinions have been offered over time regarding its precise meaning. The Congressional Research Service has stated that the weight of scholarly legal and historical opinion indicates that the term means one who is entitled under the Constitution or laws of the United States to U.S. citizenship "at birth" or "by birth," including any child born "in" the United States (other than to foreign diplomats serving their country), the children of United States citizens born abroad, and those born abroad of one citizen parent who has met U.S. residency requirements.