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Loona (commonly stylized as LOONA or LOOΠΔ; Korean: 이달의 소녀; RR: Idarui Sonyeo; Japanese: 今月の少女, romanized: Kongetsu no Shōjo, lit. 'Girl of the Month') is a South Korean girl group formed by Blockberry Creative. The group was introduced to the public through a pre-debut project that began in October 2016, where each of the 12 members were revealed in a periodic fashion by releasing a promotional single over the following 18 months. They debuted as a full ensemble with the extended play, [+ +] (2018), supported by the lead single "Favorite" and the title track "Hi High".

Name
Loona's English name is derived from the Hangul letters ㅇㄷㅇㅅㄴ, each an initial consonant in the syllabic blocks that make 이달의 소녀 (Idarui Sonyeo). When rearranged to ㄴㅇㅇㄷㅅ, it resembles LOONA in the Latin alphabet.

2016–2018: Pre-debut sub-units and Mix Nine
On October 2, 2016, Blockberry Creative announced through Naver that they would be debuting their first girl group through an 18-month-long pre-debut project. Jaden Jeong served as the group's creative director until August 2019, when he parted ways because of creative differences with the company. Between October 2016 and January 2017, four members (HeeJin, HyunJin, HaSeul, and YeoJin) were revealed; each member released a single album typically consisting of a solo track and another track performed with other revealed members, under the group's Korean name Idarui Sonyeo (Korean: 이달의 소녀; lit. Girl of the Month). In March 2017, the group's first sub-unit, Loona 1/3, was introduced, consisting of previously revealed members HeeJin, HyunJin, HaSeul, and a new member, ViVi. Loona 1/3 released the extended play Love & Live and an accompanying single of the same name on March 13, 2017, with a television promotion on SBS's Inkigayo. On April 27, they released a repackaged edition of the extended play, titled Love & Evil, accompanied by the single "Sonatine". Love & Live and its reissued edition reached number 10 and 24, respectively, on South Korea's Gaon Album Chart.

Between April 2017 and July 2017, members ViVi, Kim Lip, JinSoul, and Choerry released their single albums, continuing the same Girl of the Month pattern. In September 2017, three members, HeeJin, HyunJin, and HaSeul, auditioned for JTBC's reality show Mix Nine; HeeJin and HyunJin passed the audition stage making it onto the show, where HyunJin placed Top 18 and HeeJin placed fourth in the finale. The group's second sub-unit, Loona Odd Eye Circle, consisting of JinSoul, Kim Lip, and Choerry, released the extended play Mix & Match and its single "Girl Front" on September 21, 2017 and began promotions on Mnet's M Countdown. The English version of their song "Loonatic" was released on October 23. On October 31, they released a repackaged edition of the extended play, titled Mix & Match, with three new songs including the single "Sweet Crazy Love". Mix & Match and the repackaged edition peaked at number 16 and 7, respectively, on the Gaon Album Chart.

Between November 2017 and January 2018, they released the solo single albums for new members Yves, Chuu, and Go Won, continuing the monthly pattern along with the digital single "The Carol 2.0" sung by Yves, Vivi, and Choerry. In March 2018, they released the solo single album for the final member, Olivia Hye. On May 30, 2018, the group's third sub-unit, Loona yyxy, made their debut with the extended play Beauty & the Beat. The single "love4eva" featuring Canadian musician Grimes was released the same day with an accompanying music video. The extended play reached number 4 on the Gaon Album Chart.

On August 7, 2018, Loona released a pre-debut digital single titled "Favorite", the group's first song to feature all twelve members, accompanied by a music video focusing on the group's choreography. The pre-debut single was later revealed to be the lead single of Loona's debut EP [+ +].

2018–2019: Official Korean debut with [+ +] and [X X]
On August 20, Loona officially debuted as a full assemble with the release of the extended-play [+ +] (read as plus plus). The album included "Favorite" as the lead single while "Hi High" served as its title track. It debuted at number 2 on South Korea's Gaon Album Chart and was the second best-selling debut album by a girl group in 2018. [+ +] was repackaged as [X X] (read as multiply multiply) on February 19, 2019, with six additional tracks, including a new single, "Butterfly". On August 17, 2019, Loona made their first appearance in the United States, performing at KCON 2019 in Los Angeles. On December 13, 2019, Loona released a single called "365" as an appreciation song for their fans.