It exists. Although the spelling of it would appear to vary somewhat.
I would think that information from the following site might be considered to be somewhat authoritative.
Yoga and meditation - Search
Here is a quote from that site.
1. Paschimotanasana
(Asanas or Postures/Asanas or Postures Items)
... Paschimotanasana: Stretch the legs like sticks on the ground. Grasp the toes. Rest your forehead on the knees. THE NOMENCLATURE Paschimotanasan is derived from the Sanskrit words 'Paschima' meaning 'posterior', 'back' or 'behind'; 'Uttana' referring to stretching out' and 'Asana', of course, connoting 'Posture'. Therefore 'Paschimotana' basically means stretching out the posterior portions of your body.
Also, I found the information on this link from that site to be particularly informative.
Yoga and meditation - Paschimotanasana
It is also mentioned frequently as Paschimotanasan while performing the following search.
Paschi-motan-asan - Google Search You might like to check out the sites on the that search.
The following quotes are from the following site:
eyogateacher - A site for learning yoga online I don't know that I would call it an authoritative reference. But, at at least something along the lines for what you are looking is mentioned.
Please see that which I have bolded in red.
10-How are Yoga postures classified ?
We saw that the no of classical postures are 84 in number and they are classified in the following five broad categories :
1) Sitting Meditative Postures e.g. : Padmasan ( Lotus posture )
2) Forward bending postures e.g..
Paschi-motan-asan ( Sitting forward bend )
3) Backward bending postures e.g. . Dhanur-asan ( Bow )
4) Twisting postures e.g.. Ardha Matsyendra –asan ( Half spinal twist )
5) Balancing postures e.g.. Kak-asan ( Crow )
These five categories are only indicative in nature and we need not strictly hold on to it . So when ever we do yoga practice we must see to it that our practice has the postures of the above 5 categories . The 12 Basic postures as designed by Swami Vishnu devananda has all the postures that fit in to the above 5 category and hence is a complete system of exercise in itself .
The basis on which the postures are classified is in the way the spine is getting manipulated and exercised completely .It is not only at the spinal level but also at the muscular level the entire muscular system is toned .
The 12 Basic Postures are :
1) Headstand ( Siras –asan )
2) Shoulder Stand ( Sarva –anga-asan)
3) Plough ( Hal-asan )
4) Fish ( Matsy-asan )
5) Sitting Forward Bend (
Paschi-Motan-asan )
6) Cobra ( Bhujan-asan)
7) Locust ( Shalab-asan )
8) Bow ( Dhanur-asan )
9) Half Spinal twist ( Ardha-Matsya-asan )
10) Crow ( Kak-asan ) , Peacock ( Mayur-asan)
11) Standing Forward Bend ( Pada-Hatsa-asan )
12) Triangle ( Trikon-asan )