Ballot measure is the generic term for items that appear on ballots where voters can approve or reject them.
Ballot measures get on the ballot through very different paths. Initiative and referendum, often abbreviated as "I&R", is the catch-all phrase for ballot measures that get on ballots through a signature collection process of some kind.
Only about half of the American states allow their citizens to place a measure on the ballot through the collection of signatures.
However, every state has some mechanism through which measures of some kind can make their way onto the ballot of that state. This can include legislative referrals and constitutional amendment validation procedures, as well as bond issues or tax proposals placed before the state's voters by a vote of the state legislature.
In other words, a general election ballot in any American state might include ballot measures of one kind or another. However, only in half the states would any of those ballot measures be there courtesy of a petition drive.