semiquincentennial /ˌsɛ.miˌkwɪn.sɛnˈtɛn.i.əl/adjective 1noun 1forms 1 adjective1 •Of or relating to a 250th anniversary.[Bath, N.C. SemiQuincentennial Committee] Program for celebrating the 250th anniversary of the Incorporation of Bath, North Carolina, on March 8, 1705, October 1, 2, 3, 4, 1955. [Bath, 1955] 32 p. — 1958 noun1 •A 250th anniversary.The year 2026 will mark the semiquincentennial of the founding of the United States. formssemiquincentennials