What is a gift?
Generally, anything that has a monetary value is considered a gift.
Gift to DoD employee from outside source
With some exceptions, you may not accept a gift from anyone who is giving the gift to you because of your Government position. Ask yourself if the gift would have been offered if you were not working for the Government. If the answer is no, then the gift is being offered because of your position.
Also, you may not accept a gift from a defense contractor. You also must turn down a gift from those who have interests that may be significantly affected by your official duties.
There are many exceptions to the gift prohibition rule and exclusions to the definition of a gift. The deskbook below summarizes these exceptions and exclusions in more detail.
Gifts between employees
With a few exceptions, the general rule is that you cannot give, make a donation to, or ask for contributions for, a gift to your official superior. An official superior includes your immediate boss and anyone above your boss in the chain of command. Also, an employee cannot accept a gift from another employee who earns less pay, unless the person giving the gift is not a subordinate and the gift is based on a strictly personal relationship.
There are some exceptions to this rule such as exchanging gifts on an appropriate and customary occasion, or the exception for special and infrequent occasions, or the group gift.
These exceptions are covered in more detail in the relevant deskbook chapter.
Gifts from Foreign Government
Generally, Federal personnel may not accept gifts from foreign governments. There are some exceptions such as a gift that does not exceed the minimal value or the gift of lodging, travel and meals provided such a gift is offered while the Federal traveler is overseas. Finally, this restriction in the Constitution, (e.g. the Emoluments Clause) prohibits retired military members from accepting gifts from foreign governments unless they receive advance approval. More details can be found in the SOCO white paper.
Gifts in general
There is a variety of other gift acceptance authority covered in the Deskbook. For example, the DoD has gift acceptance authority to accept personal and real property. The General Services Administration has a government-wide statute that permits Federal personnel to accept gifts of travel, meals, lodging, and registration fees. The DoD also has specific authority to permit military and civilian members to accept certain gifts if they were in a "combat zone". Again, a more detailed description of these types of gifts and others can be found in the deskbook chapter on gifts.