The information you are looking for should be mentioned in the municipal code for your city. If not, your state likely has that information in the state statutes. In Washington, such matters fall under the Revised Code of Washington Title 42 "Public officers and agencies", Chapter 17 "Disclosure — campaign finances — lobbying." RCW 42.17.020(22) states:
"Expenditure" includes a payment, contribution, subscription, distribution, loan, advance, deposit, or gift of money or anything of value, and includes a contract, promise, or agreement, whether or not legally enforceable, to make an expenditure. The term "expenditure" also includes a promise to pay, a payment, or a transfer of anything of value in exchange for goods, services, property, facilities, or anything of value for the purpose of assisting, benefiting, or honoring any public official or candidate, or assisting in furthering or opposing any election campaign. For the purposes of this chapter, agreements to make expenditures, contracts, and promises to pay may be reported as estimated obligations until actual payment is made. The term "expenditure" shall not include the partial or complete repayment by a candidate or political committee of the principal of a loan, the receipt of which loan has been properly reported.
RCW 42.17.020(28) gives a lengthy definition of "independent expenditure" which is too long to quote, but you can find the whole of section 020
here. Again, these are for Washington elections which operate under state law.