乳児|にゅうじnyuuji[1]atamadaka (head-high)

1. nouninfant (below 1 year old); suckling baby

• Synonym赤ん坊あかんぼうCasualThe colloquial word for a baby, not restricted to under one year like 「乳児」; it can also describe someone naive about the world.
• Synonym赤子あかごWrittenA written-style word for a baby, often met in figurative set phrases such as 「赤子の手をひねる」.
• Synonym赤ちゃんあかちゃんNeutralThe most common everyday word for a baby, warm and affectionate; 「乳児」 is the formal term for infants under one.
• SynonymベビーNeutralA loanword for baby, used mostly inside compounds such as 「ベビーカー」 rather than as a standalone word like 「乳児」.
• Synonym幼子おさなごWrittenA literary word for a young child, with a wider age range than 「乳児」; its literary flavor contrasts with the clinical tone of 「乳児」.
• SynonymみどりごWrittenAn old-fashioned, literary word for a newborn baby, found mostly in literature rather than everyday or official writing.
• Synonym乳飲み子ちのみごWrittenA written word for a child still at the breast, carrying a tender, affectionate nuance that the plain 「乳児」 does not.
• Related幼児ようじNeutralCovers children from about age one up to school age, so in this classification they are older than those called 「乳児」.
• Antonym老人ろうじんNeutralAn old person or the elderly — the opposite end of the age scale from 「乳児」.
• Antonym成人せいじんNeutralAn adult, someone who has come of age — the opposite of 「乳児」 on the age scale.

Use 「乳児」 for babies under one in medical or administrative writing. In everyday speech say 「赤ちゃん」 or 「赤ん坊」; from age one to school age it is 「幼児」. 「嬰児」「赤子」「乳飲み子」「幼子」 are literary, 「ベビー」 mostly appears in compounds, and 「老人」「成人」 are the age opposites.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 生後せいご1年ねん程度ていどで母乳ぼにゅうなどで育そだてられる小児しょうにのこと。