I just downloaded Lens based on this comment and now I can't stop playing with it. I feel like a wizard!

Interesting way to phrase that

The shirt is a tubular tee shirt, and the v-shaped insert is called a gusset XD

Originally, the design was a ribbed knit material that was inserted at the collar and stitched into place. It served two main purposes: Like the ribbing at the sleeves and at the hem, the ribbed insert allowed the wearer to more easily don the garment without it losing shape. As the wearer would get their head through the neck of the sweatshirt, the V-insert would stretch to accommodate. The elasticity of the ribbing could stretch as needed without losing integrity.

The second reason is that it helped to absorb sweat. If you’ve seen someone sweat through a crewneck, you’ll know that the chest is a major point of perspiration. Though, the V-insert could only take so much.

Neat

This is one of those questions that I didn't ever think to ask but glad I know the answer to now.

I had the most success when using the terminology from this article, thanks! "V-notch insert" mostly worked (although I think it's just not a thing people really mention when describing sweaters.)

I really enjoyed reading that page, so thank you for finding and sharing it!