What are the possible words for a task?

I am looking for similar words to task for a document on scheduling tasks in the context of a project. My goal would be to find words that would denote meanings for three things.

• A word for a small atomic task of short duration that is to be done. This would be the fundamental unit. I'm thinking of: job, assignment.

• A word to denote that this small task has been done. I'm thinking of: work.

• A word for a larger task that is made of several small tasks, or even several tasks. I'm thinking of: task, labor.

As a non native English speaker, I am having troubles finding the exact meaning of all these words, and if my hierarchy, from small to large, is correct. Do native speaker have suggestions for other words?

In my software development projects I use "todo" for the smallest unit of work, "task" for the next highest, which might be comprised of several todos, and "milestone" for the next, which is typically comprised of several tasks. A project, then, is made up of several milestones.

I know these are not stylistically elegant, but they are fairly commonly understood on the software projects I've worked on.

For a [fairly] complete list, you might check out the synonyms listed here: http://thesaurus.com/browse/task

I would use "task" for small pieces of work and "project" for the work that is made up of smaller tasks. When a small task (or a project) is done, you could mark it "completed" or "finished".