semicentennial

[ sem-ee-sen-ten-ee-uh l, sem-ahy- ]
/ ˌsɛm i sɛnˈtɛn i əl, ˌsɛm aɪ- /

adjective

of or relating to a fiftieth anniversary.

noun

a fiftieth anniversary.
the celebration of this anniversary.

Origin of semicentennial

First recorded in 1855–60; semi- + centennial

British Dictionary definitions for semicentennial

semicentennial
/ (ˌsɛmɪsɛnˈtɛnɪəl) /

adjective

(prenominal) of or relating to the 50th anniversary of some event
occurring once every 50 years

noun

a 50th anniversary