A thread is a single sequential flow of control within a program. There is nothing new in the concept of a single thread. The real hoopla surrounding threads is not about a single sequential thread. Rather, it's about the use of multiple threads in a single program, running at the same time and performing different tasks. Within the HotJava browser you can scroll a page while it's downloading an applet or image, play animation and sound concurrently, print a page in the background while you download a new page, or watch three sorting algorithms race to the finish.
Some texts use the name lightweight process instead of thread.
A thread is similar to a real process in that a thread and a running program are both a single sequential flow of control. However, a thread is considered lightweight because it runs within the context of a full-blown program and takes advantage of the resources allocated for that program and the program's environment. As a sequential flow of control, a thread must carve out some of its own resources within a running program. The code running within the thread works only within that context. Thus, some other texts use execution context as a synonym for thread.