noun
opening game; season opener
Refers to the first match of a sports season or tournament. Common in baseball, soccer, and sumo contexts.
プロ野球の開幕戦は来週の金曜日です。
The opening game of the professional baseball season is next Friday.
開幕戦でいきなりホームランを打った。
He hit a home run right in the season opener.
初戦 is a more general term for 'first match' in a tournament or series, not necessarily the season opener.
Compound of 開幕 (opening, curtain-raising) and 戦 (match, battle). The exact historical derivation is uncertain, but the word is a straightforward modern sports term.