Please don't tell your students a diary is a primary source.

Diary of Wilbur Wright, 1900
https://www.loc.gov/item/wright002232/

During most of my teaching, instead of a definition for what a primary source is, I simply provided my students examples of primary sources. "What is a primary source?" "A letter," "Diary," "Photograph." But I was doing a disservice by giving a list of formats and not an actual definition.

As time went on, I used others definitions and worked to make meaning of them, but talking to educators about their definition of a primary source has helped me again evolve and move to my own definition.

Who Were The Wright Brothers
by James Buckley Jr.
and Tim Foley

I would say that a primary source is a source directly tied to a topic and a time. Take, for instance, the 1900 diary of Wilbur Wright above. You may be studying late 19th and early 20th century attempts at flight, inventors or the Wright Brothers themselves. If I were studying any of these topics, this may be an appropriate source. Let's focus on late 19th and early 20th century attempts at flight as a topic. What makes the diary a primary source is that it was also created at the time of the event I'm studying. Of course, a book, written recently about the brothers is still connected to the topic, but not the time, and therefore is a secondary source. Some other piece, for example, a letter written to George Washington by Benedict Arnold, isn't a source at all when studying this topic.

Talking with teachers about their definition of a primary source and looking back on how I taught was a primary source was, my definition reveals some common misconceptions.

Misconception #1 A source is always primary or secondary.
Just by looking at the example, you can see that my old activity of sorting by format won't work. A letter isn't a primary source because it is a letter. It may be a primary source if it is connected with my topic of study and created during the time that the topic of study took place. Sources are primary or secondary (or not a source at all) depending on the topic and time of focus.

Misconception #2 Primary sources are "fact" while secondary sources are "opinion."
This isn't shown in the example, but many teachers, when sharing their definition of a primary source or when comparing it to a secondary source had an element of this misconception. Many teachers and students will imply that primary sources are better, truer, or more factual than secondary sources. In fact, primary sources contain plenty of opinion, bias, and perspective. The creator of the source brings that to the source itself. A written primary source has a perspective, a map has boundaries that the creator has decided to focus on and others that are not shown. A primary source photograph shows a certain view of an event and does not show other aspects, reflecting the perspective and choice of the creator.

Misconception #3 Primary sources are always "first hand accounts."
This may be the most controversial of what I perceive to be misconceptions, but hear me out. When I visit major institutions that organize primary sources by topic or event such as the Library of Congress, Docsteach, Stanford History Education Group, DPLA, and others, and I look at the sources that they identify as primary sources for a specific topic. There are countless cases where the source is not a "first hand account" of the topic it is identified under. These institutions, by their actions, if not by their definitions (and they vary) show that primary sources are not always first hand accounts.

I usually think of a definition as something static and unchanging. I think I should be able to break out the dictionary from my youth and the definition I read there should suffice. That has never quite worked for the definition of a primary source. Instead, my definition has evolved from a list of formats to others' definitions to my own. It likely will continue to evolve to inform my own understanding.