Extra and Follow-on

Extra noun - An interchangeable part or piece of equipment that is kept on hand for replacement of an original.

Follow-on and extra are semantically related. In some cases you can use "Follow-on" instead a noun "Extra".

Follow-on noun - An immediate second innings forced on a cricket team scoring a prescribed number of runs fewer than its opponents in the first innings.

Extra and follow-on are semantically related. Sometimes you can use "Extra" instead a noun phrase "Follow-on".