The Sagan standard is related to astronomer Carl Sagan, who stated that “extraordinary claims require extraordinary evidence” (a dictum abbreviated as ECREE). 
This means that the more unlikely a certain claim is, given existing evidence on the matter, the greater the standard of proof that is expected of it.

The Sagan Standard

Problem Solving

Based on the Sagan standard, if someone claims that they came across a unicorn during they commute, they would be expected to brig stronger evidence in order to verify that claim than if they claimed that they came across a horse. 
This happens because there is significant evidence for the existence of horses, but no relevant evidence to support the existence of unicorns, which makes the latter claim extraordinary.

Ordinary And Extraordinary Claims

<ul>
<li>Instead of viewing claims as either ordinary or extraordinary, it’s better to view them as ranging between these two ends of the spectrum, based on how likely they are given everything that is known on the subject.</li>
<li>It can be difficult to define the exact threshold on the ordinary-extraordinary spectrum that a certain claim needs to cross before it’s considered extraordinary, it’s generally preferable to focus on how extraordinary a claim is instead, and to expect a stand of proof that matches that degree of extraordinariness.</li>
<li>A claim should generally not be viewed as extraordinary simply because it’s novel, but rather because it contradicts existing evidence.</li>
</ul>

The Concept Of Extraordinary Claims

There is no clear separation between what is ‘ordinary’ evidence and what is ‘extraordinary’ evidence; the answer is in most cases subjective, though it should nevertheless be based on sound reasoning.
However, it is possible to argue in favor or against the extraordinariness of evidence, based on its quantity and quality, and based on the relevant standards that apply to the subject being discussed.

The Concept Of Extraordinary Evidence

Overall, the concept of ECREE does not mean that claims that contradict the current scientific consensus should always be ignored, or that claims that support the consensus should be automatically accepted. Rather, ECREE simply suggests that the likelihood that a certain claim is true, based on preexisting knowledge, should be taken into account when determining how much evidence is needed in order to verify it.

Don't Simply Ignore Extraordinarily  Claims

The more unlikely a claim is, the more supporting substantial the evidence that you should require before accepting it as true.
When presented with a certain claim, you should expect the strength of evidence that is used to support it to be proportional to how unlikely that claim is, based on prior information.

Ordinary And Extraordinary Claims

Double Standards: What They Are and How to Respond to Them

The double standard

Double standard examples

Why double standards happen

The Appeal to Definition Fallacy: When People Misuse the Dictionary

The argument from dictionary

An example of the appeal to definition

Appeal to definition: when the process becomes fallacious