I am new in Unity development and I have good experience in Autocad development. Is it possible to use 3d solids & 3d mesh terrains created in autocad and display it in the
Unity supports .FBX, .dae (Collada), .3DS, .dxf and .obj files. If your Autocad can export to any of these then the answer is yes. It is possible but read below.
Objects produced in Autocad, SolidWorks or other similar software should not be imported to Unity directly by any means. Its too heavy for Unity. If you want to use your models from any of these software, export them to software such as Maya or Blender then clean the models up. Reduce the poly counts before importing into Unity.
Note that it will take time to clean the models up. You should spend that time making new models in Maya or Blender. If your goal is to make a game, then you need to learn either Maya or Blender. Autocad is not used for that. Since you are already using Autocad, I suggest you use Maya since the UIs are now made to look similar.
EDIT:
I see an answer below suggesting that it's totally fine to use Autocad models in Unity or model the objects in Autocad. It's not just about creating models. You have to do some work on the models to make it look good and even work properly with Unity's render engine. Here are many reasons not to use Autocad to model your game objects:
You have to unwrap the model's UV before importing the model into a game engine. Without this, it's hard to do texturing and you'll run into issues. Not to mention one of the biggest issue called "texture seams".
Again, this is done inside programs like Maya and Blender not Autocad.
You need to bake maps most of the times when working with high detailed models. When generating materials from V-ray or any renderer in Maya with complex materials, you need to to bake the final result into may types of maps such as color, normal, alpha.... This also applies to light baking.
Initially, you model high poly then transfer the map to a low poly. You can't do this with Autocad. This is very import as it reduces the number of polys in your game.
Everything mentioned above are very important when it comes to game development but Autocad is not made to handle those.
Use the right tool for the job. Use Autocad for engineering and Maya or Blender for games, movies and art. It's as simple as that.