Is the term avatar unique to Hinduism?
Alterntively it is used as a general term to mean different concepts?
We can examine its meaning in Sanskrit this way.
Avatara = Ava + Tara
Tara means "Brilliantly Shining Far Away Luminous Point" (~star in the sky) Ava means "Coming Down"
In other words, it is the highest/ultimate reality taking a temporary name & form in the empirical world for fulfilling a particular task and merge back (Avatara PariSamapthi) when the task is completely fulfilled.
Example: In Vaishnava Tradition when Lord Vishnu is The Supreme Eternal Reality, Rama & Krishna are his Avataras.