Bytecode, also known as p-code (portable code), is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (normally numeric addresses) which encode the result of parsing and semantic analysis of things like type, scope, and nesting depths of program objects.

Cost Analysis of Java Bytecode
Soot - a Java bytecode optimization framework
Java Bytecode Verification: Algorithms and Formalizations
A type system for Java bytecode subroutines
A type system for Java bytecode subroutines
PicoJava: a direct execution engine for Java bytecode
PicoJava: A Direct Execution Engine For Java Bytecode
A type system for object initialization in the Java bytecode language
Bytecode program interpreter apparatus and method with pre-verification of data type restrictions and object initialization
A Type System for Object Initialization In the Java Bytecode Language (summary)