A very close friend or best friend; someone with whom one has a deep, long-standing bond.
彼は大学時代の大親友で、今でも年に一度は会っている。
He is my best friend from university, and we still meet once a year.
大親友の結婚式でスピーチを頼まれ、とても緊張した。
I was asked to give a speech at my best friend's wedding and I was very nervous.
困ったときは大親友に相談するのが一番だ。
When in trouble, consulting your best friend is the best thing to do.