B1/B2 nonimmigrant visa is defined at INA 101(a)
(15) The term "immigrant" means every alien except an alien who is within one of the following classes of nonimmigrant aliens
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(B) an alien (other than one coming for the purpose of study or of performing skilled or unskilled labor or as a representative of foreign press, radio, film, or other foreign information media coming to engage in such vocation) having a residence in a foreign country which he has no intention of abandoning and who is visiting the United States temporarily for business or temporarily for pleasure;
AND admitted, generally, pursuant to 8 CFR 214.2(b)
(1) General. Any B-1 visitor for business or B-2 visitor for pleasure may be admitted for not more than one year and may be granted extensions of temporary stay in increments of not more than six months each, except that alien members of a religious denomination coming temporarily and solely to do missionary work in behalf of a religious denomination may be granted extensions of not more than one year each, provided that such work does not involve the selling of articles or the solicitation or acceptance of do nations. Those B-1 and B-2 visitors admitted pursuant to the waiver provided at § 212.1(e) of this chapter may be admitted to and stay on Guam for a period not to exceed fifteen days and are not eligible for extensions of stay. (Paragraph (b)(1) revised 1/1/94; 58 FR 69210)
(2) Minimum six month admissions. Any B - 2 visitor who is found otherwise admissible and is issued a Form I - 94, will be admitted for a minimum period of six months, regardless of whether less time is requested, provided, that any required passport is valid as specified in section 212(a)(26) of the Act. Exceptions to the minimum six month admission may be made only in individual cases upon the specific approval of the district director for good cause.
***They make exceptions for both longer and SHORTER periods of admission***
***There is more but it is probably irrelevant***
