What do you call the portion of a tail-dragger takeoff when the tail is in the air?
There is no term (that I am aware of) referring to only the portion of the takeoff while the tail is off the ground.

In general, the attitude where just the two main wheels are touching the ground is called "two-point." Likewise, with the tail wheel also on the ground, it is called "three-point."

Specifically, a two-point landing is sometimes called a "wheel landing".

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