Addendum
An official, written change to the bid documents, such as plans, specs, or bidding instructions, issued by the architect or owner before bids are due, correcting an error or adding information every bidder needs.
Why it matters
If one contractor finds out about a correction and others don't, they're no longer bidding on the same project, which isn't fair to anyone and can void the whole bid process. An addendum is what keeps every bidder working from the same, current information.
On a real project
A week before bids are due, the architect discovers a dimension error on a structural drawing. Rather than telling contractors informally, they issue a numbered addendum to every bidder correcting the drawing, and every bidder has to acknowledge receiving it in their bid.
Who this matters most to
An Estimator has to catch every addendum and fold its changes into the bid before it's submitted; missing one is one of the more common, avoidable bid mistakes. A Project Engineer often tracks addenda after award, since they become part of the official contract documents.
Where this goes wrong
An estimator finishes a bid and submits it the morning it's due without checking for a late addendum that reduced the scope of work. The bid comes in artificially high because it still includes work the addendum removed, and the contractor loses the project to a competitor who caught the change.