誕生日とは人が生まれた日のことだ。

birthday

if; when; and; with; particle used for quoting (with speech, thoughts, etc.); quoting particle; promoted pawn; indicates question (sentence end)

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

man; person; human being; mankind; people; human (Homo sapiens); humans as a species; character; personality; man of talent; true man; another person; other people; others; adult

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

to be born

day; days; sun; sunshine; sunlight; case (esp. unfortunate); event

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

thing; matter; incident; occurrence; event; something serious; trouble; crisis; circumstances; situation; state of affairs; work; business; affair; after an inflectable word, creates a noun phrase indicating something the speaker does not feel close to; nominalizing suffix; pretending to ...; playing make-believe ...; alias; aka; nickname; alternative name; also known as

be; is; indicates past or completed action; indicates light imperative