What is rigor?
Rigor is a buzzword in modern education, but what is rigor and how does it truly apply in education?
According to the Merriam-Webster dictionary, rigor is defined as strict precision or exactness. It can also be defined as the quality of being unyielding or inflexible, otherwise known as strictness. 
Strict precision or exactness in education should be the idea that a student’s learning is held to a high standard of accuracy. Unyielding or inflexibility in education should mean that a student’s learning expectations are held to that standard, even if the model or pacing of education must change in order for the child to meet that standard. However, educational leadership tends to focus this ideal on assignments rather than content. If an assignment causes students to process in a more critical or creative fashion, it is considered rigorous. It is also noteworthy how this term is applied to the learning environment. The ideal rigorous environment must be stimulating - engaging students in a supportive setting.  For many, the meaning of rigor in education means to move away from memorization and recall, into more of an analysis or interpretation of the information.
In academia, this term is often used to describe an instructional experience that is challenging to students academically, intellectually, and personally. The idea is that a rigorous learning experience will help students understand concepts that are more difficult in their complexity. Often, ambiguous and contentious topics are included as well. Rigorous work is supposed to help students gain useful educational, career, and civic skills.
In actual practice, the educational standard and teaching models have not primarily changed, and many children are not able to meet the standard that has been set. Rather, these students are simply said to have “failed” and the lesson moves forward. In this way, the expectation on the child has been unwittingly altered.
True rigor in education should never allow the expectation on a child’s learning to be set aside for the convenience of the curriculum.
Some educators argue that rigorous learning needs to be challenging at an appropriate level for each individual student. Simply making work more difficult without thought of the needs of the individual can lead to discouragement, frustration, and ultimately disengagement. Learning that is at an appropriate level of rigor is more likely to motivate students in their learning, bringing deeper and more robust learning and a strong sense of accomplishment when a learning challenge is overcome. This is the goal of mastery learning.
 (Merriam-Webster, 2020, part 4 & 1a2): https://www.merriam-webster.com/dictionary/rigor?utm_campaign=sd&utm_medium=serp&utm_source=jsonld
 Information on the standard view of rigor comes from the Glossary Of Education Reform: https://www.edglossary.org/rigor/