public sealed class TerrainDefinition : ArcGIS.Core.Data.Definition, System.IDisposable  

{
  string path = @"d:\Data\Terrain\filegdb_Containing_A_Terrain.gdb";
  var fileConnection = new FileGeodatabaseConnectionPath(new Uri(path));
  //Note: Use QueuedTask.Run to run on the MCT 
  using (Geodatabase dataStore = new Geodatabase(fileConnection))
  {
    string dsName = "nameOfTerrain";

    using (var terrainDefinition = dataStore.GetDefinition<ArcGIS.Core.Data.Analyst3D.TerrainDefinition>(dsName))
    {
      // get the feature class names that are used in the terrain
      var fcNames = terrainDefinition.GetFeatureClassNames();
    }
  }
}

{
  //Note: Needs QueuedTask to run
  {
    using (var tin = tinLayer.GetTinDataset())
    {
      using var tinDef = tin.GetDefinition();
      Envelope extent = tinDef.GetExtent();
      SpatialReference sr = tinDef.GetSpatialReference();
    }

    using (terrain = terrainLayer.GetTerrain())
    {
      using var terrainDef = terrain.GetDefinition();
      Envelope extent = terrainDef.GetExtent();
      SpatialReference sr = terrainDef.GetSpatialReference();
    }

    using (lasDataset = lasDatasetLayer.GetLasDataset())
    {
      using var lasDatasetDef = lasDataset.GetDefinition();
      Envelope extent = lasDatasetDef.GetExtent();
      SpatialReference sr = lasDatasetDef.GetSpatialReference();
    }
  }
}