Rules on a Cloud Storage bucket that apply automatic actions, such as SetStorageClass or Delete, when objects meet conditions such as age or the number of newer versions.
The typical ACE question is "move to Coldline after 30 days and delete after 365": lifecycle rules solve it with no scripts. To prevent early deletion you use a retention policy, not lifecycle rules.
Object Lifecycle Management is a concept that can show up on the exam. Check the full guide to see it in context.