Lambda the Ultimate

Theta is a sequential, strongly-typed, object-oriented language. It provides separate mechanisms for type hierarchy, inheritance, and parametric polymorphism. It also provides separate mechanisms for specifications, which define the interfaces of new abstractions, and code that implements the new abstractions, and it allows multiple implementations of types and routines. It has a module mechanism that encapsulates the details of type and routine implementations, while allowing related implementations to share implementation-specific information. Theta is largely derived from CLU, but has also been influenced by Trellis/Owl, Modula-3, C++, and Emerald.

One more for the single-letter languages list. A product of MIT's Programming Methodology Group. Developed for this group's Thor system:

Thor is a large-scale distributed, object-oriented database system that provides reliable and highly available persistent storage for objects....

Thor allows application programmers to use their preferred programming language and even to develop applications using a variety of programming languages; the components in the different languages will be able to inter-operate by sharing objects in Thor.

The objects in Thor are specified and implemented in Theta, a new programming language developed by the Programming Methodology Group.