Awesome
Understanding Mathematics 
Articles
- Understanding Mathematics, a study guide by Peter Alfeld
- How to Develop a Mindset for Math at BetterExplained
- Understanding Math Concepts at AllKindsOfMinds
- What is conceptual understanding? by Keith Devlin
- Learning to Learn: Math Abstraction at BetterExplained
- How to study math by Paul
Papers
- Teaching Mathematics for Understanding by Van De Walle
- Can We Make Mathematics Intelligible? (1981) by R. P. Boas
- A Mathematician's Lament by Paul Lockhart
- How to Survive Your College Math Class (and Take Home Something of Value) by Matthew Saltzman and Marie Coffin
- Mathematics, morally by Eugenia Cheng
Books
- How to Solve It: A New Aspect of Mathematical Method by George Polya
- Measurement by Paul Lockhart
- A Mathematician's Lament by Paul Lockhart
- The Art and Craft of Problem Solving by Paul Zeitz
Videos
- Mathematics - The Language of the Universe
- The World of Mathematical Reality by Paul Lockhart
- Paul Lockhart teaching Go by Chaelim "Addy" Kim
- Five Principles of Extraordinary Math Teaching TEDx talk by Dan Finkel
Online Questions
- How to study math to really understand it and have a healthy lifestyle with free time? at Math StackExchange
Mathematical Language
Links to resources talking about "the mathematical language" and how to overcome problems encountered when reading, writing, interpreting and listening to mathematics.
Articles
- Language of mathematics at Wikipedia
- Mathematical Words: Origins and Sources by John Aldrich
- Communicating in the Language of Mathematics by David Moursund and edited by Dick Ricketts
- How to write mathematics clearly by Matthew Leitch
- How to understand math formulas at Interactive Mathematics
- The symbolic language of math by Charles Wells
Papers
- The Language and Grammar of Mathematics by Timothy Gowers
- Reading Mathematics by John Hamal Hubbard
- A Guide to Writing Mathematics by Dr. Kevin P. Lee
- Mathematics and the Language of Nature by F. David Peat
- Learning the Language of Mathematics by Robert E. Jamison
Books
- Handbook for Spoken Mathematics by Lawrence A. Chang et al.
- A Handbook of Mathematical Discourse by Charles Wells
Videos
- Math isn't hard, it's a language TEDx talk by Randy Palisoc
Mathematical Notation & Terminology
Articles
- History of mathematical notation at Wikipedia
- Mathematical and scientific symbols at http://www.uefap.com/
- LaTeX Math Symbols prepared by L. Kocbach (on the basis of another document by David Carlisle, Manchester University)
Wikipedia
Symbols
- List of mathematical symbols by subject
- List of mathematical constants
- List of mathematical symbols
- Mathematical operators and symbols in Unicode
Notation
- Mathematical notation
- Mathematical notation (category)
- Set notation
- Notation in probability and statistics
- Notation for differentiation
Glossaries
- Glossary of probability and statistics
- Glossary of calculus
- Glossary of set theory
- Glossary of linear algebra
- Glossary of graph theory
- Glossary of game theory
- Glossary of mathematical symbols
- Glossaries of mathematics
Papers
- Mathematical Operators at http://unicode.org
- Some Common Mathematical Symbols and Abbreviations (with History) by Isaiah Lankham, Bruno Nachtergaele, Anne Schilling
Books
- Mathematical Notation: A Guide for Engineers and Scientists by Edward R. Scheinerman
- The Cambridge Dictionary of Statistics by B. S. Everitt and Anders Skrondal
Videos
- Mathematical Notation - The Language of Mathematics by Sarada Herke
Slides
- Maths in Lectures: Understanding the Notation by Dr. David Butler
Online Questions
- All questions about mathematical notation sorted in descending order of votes at Math StackExchange
- All questions about mathematical notation sorted in descending order of votes at Stack Overflow
- All questions about mathematical terminology sorted in descending order of votes at Math StackExchange
- Where can I learn about mathematical notation? at Math StackExchange
- How to become comfortable with mathematical notation? at Ask MetaFilter
- What is the status of generating LaTeX from handwriting at Tex StackExchange
Websites
Github
- math-as-code by Jam3 (Github's repository)