An antenna is classified as Massive MIMO if it contains a large number of radiating elements (typically more than eight) operating as a single unit. Massive MIMO antennas typically comprise of an antenna array, such as 64T64R, 128T128R, 256T256R.
Single unit is an arbitrary term which may refer to any aspect of the antenna's operation, including but not limited to, operating frequency band, described application, or multiple element technology.
The classification is used when it is impractical or unnecessary to define individual element polarisations. Massive MIMO antennas may include any permutation of polarisation configuration, including single-polarised and dual-polarised arrays.