Wikipedia is not reliable enough to act as a primary resource for significant claims in an argument. There are some limited situations where it is appropriate, but when in doubt, do not cite it.
This is not intended as an attack against Wikipedia. Used correctly, it is a useful tool for starting research and finding other sources that you can reference. It is frequently correct. However, it sometimes relies on other resources that we would not accept as reliable. It is not the only source that we recognise as both useful and unreliable (see Other SourcesOther Sources) but has been raised as a question often enough to deserve special mention.
Situations where it is acceptable to reference Wikipedia:
When you are not making a claim, but using it to explain a technical or jargon term that some of the lay-readers may not be familiar with.
When the nature of the claim you are making is clearly uncontroversial, and is not the key aspect being investigated, e.g. the melting point of some chemical. However, if your only references are to Wikipedia, this should be a warning that your answer does not rely on empirical evidence.
When you want to demonstrate the notability of a controversial claim in a question. i.e. you are questioning something you see in Wikipedia. In such cases, changes to the page may invalidate the reference, so provide a permalink, i.e. one with an
oldid
field, like so:
en.wikipedia.org/w/index.php?title=Great_Moon_Hoax&oldid=419939762