乳児（にゅうじ、nyuuji）
- infant

The Japanese word 乳児, pronounced にゅうじ (nyuuji), refers to an infant or baby. Specifically, it describes a child from birth up to around one year old. This term focuses on the child's stage of development and their reliance on their parents or caregivers. The word emphasizes the period when a baby is primarily nourished by milk (from the mother or formula). It's a formal and common term used in various contexts, including medical settings, official documents, and everyday conversation.

⚫︎乳児の健康診断を受けました。 Translation: I had a health checkup for my infant. ⚫︎その病院は乳児のケアに特化しています。 Translation: That hospital specializes in infant care. ⚫︎乳児はまだ言葉を話せません。 Translation: Infants can't speak yet.

This word is appropriate in various situations where you're discussing babies or infants. You can use it in formal or informal conversations. It's often used when talking about medical care, developmental stages, or simply referring to a baby. It is common for writing like documents or reports.

乳児 is most commonly written with the kanji 乳 (meaning "milk" or "breast") and 児 (meaning "child"). The reading is always にゅうじ (nyuuji). The hiragana reading にゅうじ is used in situations where kanji is not appropriate or for learners who haven't yet learned the kanji. There are no variations in okurigana for this word.

⚫︎ 赤ちゃん (akachan): baby, infant (more casual and widely used) ⚫︎ ベビー (beebii): baby (loanword from English, common in casual settings) ⚫︎ 乳幼児 (nyuuyouji): infant and toddler (this encompasses a wider age range, usually up to 3 years old)

⚫︎ 大人 (otona): adult ⚫︎ 子供 (kodomo): child (this can be broader and include older children)

私の妹は最近、乳児］を出産しました。 ・Translation: My sister recently gave birth to an infant. ・Explanation: This sentence uses 乳児 to refer to the newborn baby.

乳児］の成長は目まぐるしい。 ・Translation: The growth of infants is rapid. ・Explanation: This sentence highlights the quick development that babies experience.

The word 乳児 is a compound word. 乳 (nyuu) refers to "milk" and by extension "breast" or "nursing", reflecting the primary source of nourishment for a young infant. 児 (ji) means "child". Therefore, 乳児 literally means "milk child" or "nursing child", reflecting the child's stage of dependence on milk and care during their first year.