Since you asked for a straight definition of these terms, I began by giving you MY own recollections from two years of Special Education instruction. I thought about an administrator (in regular ed.) who kept the DSM-III as close as possible to his desk. I have put together a conglomerate of sources from APA, DSM-IV, and the Mayo Clinic. I think you will find these definitions very complete, and you may paraphrase from them to create your own answer without citations.
First, the terms themselves give some clues before combining them into diagnosed disorders.
Stress: "The pattern of specific and nonspecific responses an organism makes to stimulus events that disturb its equilibrium and tax or exceed its ability to cope."
from American Psychological Association, retrieved from:
Ability to adapt to environment, emotionally, physically and socially. Those ...
A collection of professional definitions of psychological disorders, citing Mayo Clinic, DSM-IV and personal experience as sources. Stress and anxiety disorders are defined separately and as compounded problems.