A party who files an appeal is called an "appellant", "plaintiff in error", "petitioner" or "pursuer", and a party on the other side is called an "appellee", "defendant in error", "respondent".
ɐ pˈɑːɹɾi hˌuː fˈaɪlz ɐn ɐpˈiːl ɪz kˈɔːld æn "ɐpˈɛlənt", "plˈeɪntɪf ɪn ˈɛɹɚ", "pətˈɪʃənɚ" ɔːɹ "pɚsˈuːɚ", ænd ɐ pˈɑːɹɾi ɔnðɪ ˈʌðɚ sˈaɪd ɪz kˈɔːld æn "ɐpˈɛliː", "dᵻfˈɛndənt ɪn ˈɛɹɚ", "ɹᵻspˈɑːndənt".