projection

a set $\text{[math]}$ is a projection of some relation $\text{[math]}$ if $\text{[math]}$ (i.e. geometrically $\text{[math]}$ is the projection of the two-dimensional relation $\text{[math]}$ onto the $\text{[math]}$ -axis).
a set $\text{[math]}$ is in $\text{[math]}$ -form, abbreivated $\text{[math]}$ is $\text{[math]}$ , if $\text{[math]}$ is the projection of some computable relation $\text{[math]}$ .

[cite:;taken from @computability_soare_2016 chapter 2.1 characterizations of c.e. sets]