It loosely follows the same metaphor as the T-shaped pipe connector that 'tee' is named for.

    $ seq 3 | tee somefile
    1
    2
    3

            ╔═somefile
    seq═tee═╣    
            ╚═stdout


    $ seq 10 | pee 'head -n 3' 'tail -n 3'
    1
    2
    3
    8 
    9
    10


            ╔═head═╗
    seq═pee═╣      ╠═stdout
            ╚═tail═╝