1A conversational phrase used when suddenly remembering something or shifting to a new topic — roughly "that reminds me" or "speaking of which." Usually written in kana.
そういえば、昨日駅で田中さんを見かけたよ。
Oh, that reminds me — I saw Tanaka at the station yesterday.
そういえば、今日は燃えるゴミの日じゃなかった？
Come to think of it, isn't today the day for burnable trash?
そういえば、来月会社の飲み会があるらしいよ。
Speaking of which, apparently there's a company get-together next month.
そういえば、彼女に返すはずの本、まだ返してなかった。
Now that you mention it, I still haven't returned the book I owe her.