This is the 13984th most frequent Russian word.
объединившего
"Uniting" or "having united."
In this sentence, "объединявшего" is a participle derived from the verb "объединять" and modifies "движения", explaining its role of uniting people.
Он был лидером движения, объединявшего тысячи людей.
He was the leader of the movement, uniting thousands of people.
Here, "объединявшая" serves to characterize the ceremony as the subject that combines traditions.
Церемония, объединявшая различные традиции, прошла успешно.
The ceremony, uniting various traditions, went successfully.
In this example, "объединявший" describes the project as something that consolidates the efforts of specialists.