memento
An object kept to help you remember something special.
A memento is an object you keep to help you remember a particular person, place, or experience.
When your grandmother gives you her old watch before you leave for college, that watch becomes a memento of her and your time together. When you collect a smooth stone from a beach vacation, you're keeping a memento of that trip. A ticket stub from your first concert, a postcard from a friend who moved away, a team photo from your championship season: these are all mementos.
What makes something a memento isn't how valuable it is, but what it helps you remember. A plain seashell might be worthless to someone else but precious to you because it reminds you of the day you explored tide pools with your best friend. The object itself becomes a physical connection to a memory you don't want to forget.
People often keep mementos in special places: a box under the bed, a shelf in their room, or in a scrapbook. Looking at these objects years later can bring those memories flooding back, making you feel like you're right back in that moment.