A learning object is "a collection of content items, practice items, and assessment items that are combined based on a single learning objective" - Wikipedia 
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Learning objects go by many names, including content objects, chunks, educational objects, information objects, intelligent objects, knowledge bits, knowledge objects, learning components, media objects, reusable curriculum components, nuggets, reusable information objects, reusable learning objects, testable reusable units of cognition, training components, and units of learning.
The Experience API (xAPI), also known as the Tin Can API is an e-learning software specification that allows learning content and learning systems to speak to each other in a manner that records and tracks all types of learning experiences - WikiPedia
Learning Objects have a structure in the Viral Academy that was motivated more by visual design, time constraints, and a desire to make the content created mutable, portable, and easy to create.
Some Links
There is a good description over at - EduTopia .
History
The term is credited to Wayne Hodgins when he created a working group in 1994 bearing the name though the concept was first described by Gerard in 1967.
Critisism
Here is a quote from The Reusability Paradox : While we would not put the instructionally meaningful assembly of any two learning objects outside the realm of human capability, our purpose in this paper is to demonstrate that the instructional use or combination of certain types of learning objects (types here being variations of the objects grain sizes) cannot be automated. This single revelation will have a variety of meaningful implications. html