You can find the answer to your question in the Glossary of the PMBOK Guide. Here is my explanation:
A Milestone is a "significant point or event in a project, program or portfolio". A Milestone is usually displayed on a project SCHEDULE.
An Objective on the other hand is "something toward which work is to be directed, a strategic position to be attained, a purpose to be achieved, a result to be obtained, a product to be produced or a service to be performed." Another word for objective is "Goal". An objective is usually included in the Project Charter.
The following user(s) said Thank You: Mohammed Almohimeed
Until Next Time,
Cornelius Fichtner, PMP, CSM
President, OSP International LLC
Moderators: Yolanda Mabutas, Ahmed Amin, Scott Gillard, Mary Kathrine Padua, ERIC BARTLETT, Gail Freedman, Kevin Nason, Steven Mudrinich, PMP, Mark Wuenscher, PMP, John Wolverton, Tracy Shagnea, PMP, Jada Garrett
This interview with Simona Fallavollita (LinkedIn Profile) was recorded at the magnificient Project Management Institute (PMI)® Global Conference 2017 in Chicago, Illinois. We discuss the how, what, why and when of the changes that are coming to the PMP exam.