Learned something new today:
Blittable types are data types in software applications which have a unique characteristic. Data are often represented in memory differently in managed and unmanaged code in the Microsoft .NET framework. However, blittable types are defined as having an identical presentation in memory for both environments, and can be directly shared. Understanding the difference between blittable and non-blittable types can aid in using COM Interop or P/Invoke, two techniques for interoperability in .NET applications.
[ wikipedia ]