This page in a nutshell: If an ambiguous term cannot be avoided, then the context must make its meaning clear.
In general terms, a word is ambiguous if its intended meaning is in some way unclear to the reader. There are three main reasons why this can happen:
The meaning of the word is imprecise or open to more than one interpretation. For example, in "the Sun is bright", 'bright' is a relative term that does nothing to inform the reader of how bright the Sun actually is, nor how bright it is compared to other celestial bodies. Editors should always avoid using terms such as these, except in quotations.
Some words have multiple interpretations and have different meanings dependent upon one's perspective. What one source describes as a 'war', may be described as an 'invasion' by the other side. Use of such words tends to be seen as advocating the views of one side over the other, unless they are clearly attributed to the correct side. Rather than "this is a war", state that it is viewed as a war, and who views it as such, providing suitable references. For complete neutrality, the opposing view should also be mentioned and cited, with due weight given to each side.
Words with multiple definitions tend to cause the greatest problems, because the individual definitions may not be ambiguous. The ambiguity arises because the reader may not be certain as to which definition is intended by the editor. In such cases, always provide sufficient context or explanation to make it clear to any reader which definition is intended.
In some cases wording can be ambiguous although the words are not. For example, it is common to explain an unfamiliar term by using "or" and a familiar synonym in parentheses: "the orca (or killer whale) ...". To someone unfamiliar with the subject this can be ambiguous, suggesting an alternative; compare the valid sentences "A seal pup can be eaten by an orca (or killer whale)." and "A seal pup can be eaten by an orca (or polar bear)." A clearer alternative is to omit "or": "A seal pup can be eaten by an orca (killer whale)."