NOVA was one of the biggest (maybe the biggest) English Schools in Japan. They made it easy for native English speakers to come to Japan and teach English. Many of the teachers only stayed for 1, 2, or 3 years. There are many people that used to teach for NOVA. Since they no longer do, they are called ex-NOVA teachers (ex- meaning former).
NOVA employed about 4000 native English speakers throughout the country. Earlier this year, they quit paying their teachers. Soon after that, they went bankrupt and closed down. That put about 4000 teachers out of work so there are even more ex-NOVA teacher now. They also did not refund the students who were required to prepay.
Another company has now bought the remains of NOVA and have hired back a large number of the teachers that wanted to stay.