To be honest, I've never heard anyone claim that the definition of significance is a basic fact, unless they are simply using a thesaurus. I can offer a good answer from the perspective of a History teacher in the UK; this may not be exactly what you are looking for (then again, you are a student teacher?). However, it's worth bearing in mind that there is no broad consensus on what qualifies as historically significant, as your criteria could vary wildly depending on your period of study, area of interest, and reasons for studying it in the first place.

The most basic way of thinking about historical significance is 'something that is important in world history, and therefore worth studying.' But that just plays the synonym game between 'significant' and 'important'. In order to judge significance, you must have a way of judging it. Any good historian that wishes to stake a claim on her book covering significant events will clearly explain why this is the case. However, students generally fail at this task if you just tell them to explain why something should be studied. This has led to several teachers/educational researchers developing their own criteria. Here are a few of the most common ones that have gained traction as general toolkits:

Partington had one of the first notable criteria sets in the 1980s

• Important to people living at the time
• How profoundly it affected people's lives
• How many people were affected by it
• How long people's lives were altered
• How relevant the event is to understanding the present

This is a rather method due to its holes (compared to below), but it neatly emphasises the fact that an event could have had a significant impact at the time in many different ways.

Rob Phillips (2002): The GREAT Criteria

• Groundbreaking; to what extent was it an unprecedented or unexpected event?
• Remembered; to what extent has this been remembered by subsequent groups?
• Effects that are far-reaching; essentially a more basic version of Partington's model
• Affecting the future; as above, but focused more on subsequent groups
• Terrifying; terrifying does not have to be negative (I rather suspect Philips was stretching for a good 'T' word here).

I personally prefer and use Counsell's 5 R's (2004)

• remarkable; it is striking or otherwise worthy of attention
• resulted in change; as with Partington/Phillips
• remembered; importantly, Counsell stresses it could have been remembered by people later, but not today. This helps account for the changing nature of significance.
• resonant/had 'ripples'; even if it is not remembered, it has had an impact on wider life/culture/thought. Counsell uses the example of the phrase 'rearranging the deck chairs on the Titanic'.
• revealing; studying this reveals something interesting about the period, especially something that one might not otherwise have appreciated/understood.

My favourite example for this is Martin Luther. Of course, he clearly resulted in widespread change (clear impact on Protestant Reformation), a remarkable feat given both the reach of the Catholic Church at the time, and striking for the fact that he had not intended to kickstart a new religion at all. Due to this, he has been widely remembered, especially in Year 7 classrooms across England. As far as I can tell, the notion of hammering something to the church door in most people's minds likely owes itself to the fact that Luther did this in Wittenberg (even though this was a common occurence), and of course his life inspired Martin Luther King Sr to change his and his son's names. Finally, given that Martin Luther was an Augstinian friar and Professor of Theology, most people would tend to ascribe quite a dull personality to him. However, his 1540 letter to his wife detailing that "I eat like a Bohemian and drink like a German, thanks be to God for this. Amen" reveals much about what life could be like for a man of God in Renaissance Europe.

Hope this helped!