Good question. I checked Blacks Law Dictionary and i think the two are slightly different.
The amendment is the actual change to the contract or document; the rider is the "schedule or small piece of paper reflecting an amendment, addition, or endorsement annexed to some part of a contract..."
The confusion comes, i believe, in use within industry. Lawyers use the word amendment for the change and the document reflecting the change in the original agreement. Insurance people use rider to reflect a change in the policy, i.e. insurance contract.
Maybe a little clearer?
Jun 28, 2010