noun
Formal or elevated word for a person who has authority over others: a master, lord, employer, ruler, or head of a group. Modern everyday Japanese often uses more specific words such as 雇い主, 主人, or 指導者 depending on context.
武士は主に忠義を尽くした。
The samurai showed loyalty to his lord.
彼は小国の主として民を守
As the ruler of a small country, he protected his people.
noun
the Lord; God
Christian usage referring to the Lord, especially in prayers, hymns, and religious writing.
主よ、私たちをお守りください。
Lord, please protect us.
noun
main thing; chief part; primary aim
Used for what is central or principal, especially in formal written-style expressions such as 主とする and 主たる. Do not confuse this reading しゅ with 主な, normally read おもな.
この講座は会話練習を主とする。
This course centers on conversation practice.
安全の確保がこの計画の主たる目的だ。
Securing safety is the primary aim of this plan.
Specifically means a monarch or sovereign ruler; 主 by itself is broader and more literary or formal.
Sino-Japanese reading of the kanji 主, conventionally associated with meanings such as master, lord, and principal or main. No historical derivation is specified.