In order to express that something is only one thing from within a larger category, the construction ～は～の一（ひと）つだ will be used. In this expression, (A), the thing that there is one of, will be followed by は. After this, (B) + の一（ひと）つだ will highlight the broader category to which (A) belongs.

• アンドロイドはスマートフォンのOSの一（ひと）つだ。
  Android is one of (many) smartphone operating systems.
• 電車（でんしゃ）は乗（の）り物（もの）の一（ひと）つです。
  Trains are one of (many) vehicles.

In this grammar point, の一（ひと）つ is a standard counter. This means that other counters can and will be used, when they are more appropriate than 一（ひと）つ.

• 彼（かれ）も家族（かぞく）の一人（ひとり）だ。
  He is also one member of my family.
• トマトはフルーツの一種（いっしゅ）だ。
  Tomatoes are one of many types of fruits.
• このクレヨンはこのセットの一本（いっぽん）だ。
  This crayon is part of this set.