The project management term invitation to bid is also referred to by the easy to remember three letter anagram IFB. It refers specifically to the instance in which the project management team may solicit external vendors and or third parties of particular qualification to submit their proposals and or estimates as to the completion of a particular phase or aspect of the project as a whole. The term invitation to bid can also be known as a similar term which is request for proposal, although when used the term request for proposal often denotes and refers to the submission of a longer, more formal type of report while invitation to bid can refer in a more general sense to any invitation to bid in any format. It is important to keep in mind that in some instance the term of invitation to bid can in fact refer to something even more specific depending on the particular application area. However, in general when this term is seen it is typically safe to assume that the general definition does in fact apply.
This term is defined in the 3rd and the 4th edition of the PMBOK.