To become smaller, or to make something smaller in size, importance, or strength.
“Staff morale has diminished since the new schedule was introduced.”
“The company's profits diminished after losing its main client.”
In Reading Part 1, 'diminish' can describe a decrease mentioned in a letter, such as reduced hours, benefits, or satisfaction. Watch for it in questions asking what has changed or worsened according to the writer.
Synonyms: decrease, lessen, reduce