Anison or Anisong is a music genre mostly found in the japanese industry. The word is a combination of anime and song.
Anison songs tend to have catchy vocal lines, an electronic beat and/or strings to make for "epic" moments. Songs sometimes are supposed to sound cute, which is accomplished by singing the song in a certain vocal-style reminding of children singing, cute lyrics and/or an arrangement full of high and simple lines.
Anison usually refers to songs written, produced and sang for an anime(opening and ending tracks or character songs for example), but can also be seen as music genre, talking about songs that sound like the typical song composed for an anime. Many of Nana's songs can be considered anison by one or both definitions.