Avaz version 6.6 +
If you delete a category, you can quickly undo it (within 3-5 seconds) from the Undo option as shown in the screenshot below. Please note that this option doesn't last longer.
Scenario: You are deleting a word from Edit mode.
Screenshot 1: Menu > Edit words > (Select the word)> Delete.
Screenshot 2: Tap on "undo" on the bottom right corner, within seconds, to get the word back.
Avaz version below 6.6.
If a category is deleted, you cannot undo it. However, if you have taken a Backup or Export, then you can recover it from that. Otherwise it is lost forever.
If you have a backup, you can restore from the backup. Note that the backup restores the entire vocabulary.