You are considered a re-entering UCLA student if you have not registered for two or more consecutive quarters. Undergraduate students are not required to apply for readmission if they are absent from the University for only one term.
If you are a re-entering student, you need to fill out an Application for Readmission with the Registrar's Office. Do not fill out the UC Undergraduate Application for Admission and Scholarships (the application form you originally used for first-time admission to UCLA).
Contact the Registrar's Office readmission section at (310) 825-1091 for further information.