Are meso compounds also included in optical isomers or are they just stereoisomers

Our teacher has taught us today about optical isomers . And at the end given a formula for finding number of optical isomers of a symmetrical molecule( n is number of chiral atoms and the n is odd for this formula ). Which is
Total = Active ( referring to enantiomers ) + Meso.
And Total = 2^n-1


I am a bit confused here because I have never imagined meso compound as optical isomers for they are optically inactive and show no deflection for polarised light. So are they optical isomers. Is my definition of optical isomers flawed. Please enlighten me . Thanks in advance. I know that they are stereo isomers for their spatial arrangement in 3D. Please help me in thinking in the right direction.


