Would it be a spi using the following definition ?

The girth of the spi at midheight (line from mid luff to mid leech) = 75 % of the foot of the spi sail.

If so than it is a spinnaker or rather asymmetric spinnaker by the definition of the ISAF. If not than it is something else and often it will be a hooter or Genua.

Wouter