Looking at Wikipedia for the definition of a task (http://en.wikipedia.org/wiki/Task), I found two of them. Here they are:
In project management, an activity that needs to be accomplished within a defined period of time
In computing, a program execution context
Some examples of tasks in Drupal projects are clearing caches, indexing content into a search engine, or importing content from a third-party API. These can be classified in the following types:
Drush is really good at running long tasks in an isolated process. It supports both Batch and Queue APIs, so the workload can be either split into batches or workers, respectively. In this chapter, we will see some tips and examples of best practices to run tasks against a Drupal project using Drush. Here are the main topics...