označiti

1. to mark; to label
   Označio sam markerom najvažnije delove teksta. — I marked the most important parts of the text with a highlighter.
   Staza je dobro označena, nećete se izgubiti. — The trail is well marked, you won't get lost.
2. (of an event) to mark; to signal
   Taj gol je označio kraj njegove karijere. — That goal marked the end of his career.
3. (on social media) to tag
   Označila me je na slici sa žurke. — She tagged me in a photo from the party.
4. (+ kao) to describe as; to designate as
   Stručnjaci su ovu godinu označili kao najtopliju do sada. — Experts described this year as the warmest so far.

The imperfective is označavati; to celebrate an anniversary is usually obeležiti rather than označiti.