10 U.S. Code § 101 states:
"The term “active duty” means full-time duty in the active military service of the United States. Such term includes full-time training duty, annual training duty, and
attendance, while in the active military service, at a school designated as a service school by law or by the Secretary of the military department concerned. Such term does not include full-time National Guard duty."
38 U.S. Code § 1965, in the context of veteran's benefits, says something similar:
"The term “active duty” means—
(A)
full-time duty in the Armed Forces, other than active duty for training;
(B)
full-time duty (other than for training purposes) as a commissioned officer of the Regular or Reserve Corps
[1] of the Public Health Service;
(C)
full-time duty as a commissioned officer of the National Oceanic and Atmospheric Administration; and
(D)
full-time duty as a cadet or midshipman at the United States Military Academy, United States Naval Academy, United States Air Force Academy, or the United States Coast Guard Academy."
Additionally,
38 Code of Federal Regulations § 17.31 states:
"
Active duty for training means:
(1) Full-time duty in the Armed Forces performed by Reserves for training purposes.
(2) Full-time duty for training purposes performed as a commissioned officer of the Reserve Corps of the Public Health service during the period covered in paragraph (b)(2) of this section.
(3) In the case of members of the Army National Guard or Air National Guard of any State, full-time duty under sections 316, 502, 503, 504, or 505 of title 32 U.S.C., or the prior corresponding provisions of law.
(4) Duty performed by a member of a Senior Reserve Officers' Training Corps program when ordered to such duty for the purpose of training or a practice cruise under chapter 103 of title 10 U.S.C. for a period of not less than four weeks and which must be completed by the member before the member is commissioned.
(5) Attendance at the preparatory schools of the United States Air Force Academy, the United States Military Academy, or the United States Naval Academy by an individual who enters the preparatory school directly from the Reserves, National Guard or civilian life, unless the individual has a commitment to service on active duty which would be binding upon disenrollment from the preparatory school.
(6) Authorized travel to or from such duty as described in paragraph (c) of this section if an individual, when authorized or required by competent authority, assumes an obligation to perform active duty for training and is disabled from an injury, acute myocardial infarction, a cardiac arrest, or a cerebrovascular accident incurred while proceeding directly to or returning directly from such active duty for training. Authorized travel should take into account:
(i) The hour on which such individual began so to proceed or to return;
(ii) The hour on which such individual was scheduled to arrive for, or on which such individual ceased to perform, such duty;
(iii) The method of travel employed;
(iv) The itinerary;
(v) The manner in which the travel was performed; and
(vi) The immediate cause of disability."