正式|せいしきseishiki[0]heiban (flat)

1. na-adjective, no-adjectiveformal; official; regular; proper; legal; due (formalities)
   彼女かのじょの服装ふくそうは正式せいしきなパーティーでは場違ばちがいだった。↗

   Her clothes were out of place at a formal party.

• SynonymオフィシャルNeutralA loanword for official, formally announced things — 「オフィシャルサイト」.
• Synonym本式ほんしきNeutralThe full, unabbreviated orthodox form, as in 「本式の茶会」. Fairly uncommon; 正式 is what you would normally say.
• Synonym公式的こうしきてきWrittenMainly written prose. Means official, but also formulaic and rigid — 「公式的な回答」; 正式 has no such stiff nuance.
• Related本格的ほんかくてきNeutralGenuine, orthodox or fully under way — 「本格的なイタリア料理」「本格的に始まる」. The emphasis falls on degree, not on proper procedure.
• Related硬いかたいNeutralCovers stiffness: a strained atmosphere or stilted writing. 正式 is about conforming to proper procedures, not about being stiff.
• Related堅苦しいかたくるしいNeutralStiff and overly bound by etiquette, with an uncomfortable feel; 正式 itself carries no praise or blame.
• Related窮屈きゅうくつNeutralFeeling constrained and ill at ease, or literally cramped for space; it overlaps with 正式 only in the stiff, constrained sense.
• Related型通りかたどおりNeutralDoing things by the book or by custom, often with a sense of merely going through the motions — 「型通りの挨拶」.
• RelatedしかつめらしいWrittenAn affected, overly solemn air, used with a teasing tone; belongs mostly to written prose. 正式 simply states formality.
• Related慣習的かんしゅうてきWrittenFollows custom or convention, as in 「慣習的に行われる」, whether or not there is a formal basis; mainly written prose.
• Antonym略式りゃくしきNeutralA simplified form that skips the proper procedures — 「略式の礼服」. The direct opposite of 正式.
• Antonym非公式ひこうしきNeutralUnofficial and informal — 「非公式な会談」. Stands against both 正式 and 公式.
• Antonym仮かりNeutralTemporary or provisional, a stopgap — 「仮契約」「仮登録」. The counterpart of 正式契約.
• Antonym内々うちうちNeutralHandled privately among insiders — 「内々に決める」. The opposite of open, formal proceedings.

Use 「正式」 for something done by proper procedures. For official announcements say 「公式」 or 「オフィシャル」; for something genuine and full-scale, 「本格的」. The other near-synonyms add shades of stiffness or convention, while 「略式」 (simplified) and 「仮」 (provisional) are the main opposites.

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

1. 正ただしい方式ほうしきの事こと。
2. 正まさしく認みとめられる様子ようす。