When I started using three.js, that’s exactly what I fell for. 
Similar terms for different facts are always a problem. Wouldn’t it be appropriate to specify the designations?
How about
Object3DGroup or ObjectGroup
and
.materialGroups
.addMaterialGroup
to prettify the library?
Matching the suggestions THREE.MeshGeometry, THREE.LineGeometry and THREE.PointsGeometry @mrdoob.
THREE.Geometry will be removed from core with r125 - #15 by mrdoob
?