<p>This link contains a list of subfields: <a href="http://arxiv.org/corr/home" rel="nofollow noreferrer">http://arxiv.org/corr/home</a>, I won't reproduce them here as the link may change, and it would be redundant.</p>

<p>Also, I'm reminded of the quote of someone, can't remember who, along the lines of:</p>

<blockquote>
  <p>Mathematics is whatever
  mathematicians do</p>
</blockquote>

<p>It would seem to apply.</p>
What is the technical definition of theoretical computer science? What subfields are included?
<p>What is the technical definition of theoretical computer science?  (Or, what should it be?)</p>

<p>What main subfields does it include, and what is the commonality that separates them from the rest of computer science?</p>

<p>More specifically: if some particular research has direct practical motivations, goals and outcomes but mostly involves very abstract methods, is it theoretical computer science or not?  </p>

<p>Two examples to consider:    </p>

<p>"Dual quaternions for rigid transformation blending" (Better mathematical representation of rotation and transform for animation)
<a href="https://www.cs.tcd.ie/publications/tech-reports/reports.06/TCD-CS-2006-46.pdf" rel="nofollow noreferrer">https://www.cs.tcd.ie/publications/tech-reports/reports.06/TCD-CS-2006-46.pdf</a>  </p>

<p>"Relational Semantics for Effect-Based Program Transformations
with Dynamic Allocation" (Complier optimisation via denotational semantics):  <a href="http://research.microsoft.com/pubs/67977/ppdprelational.pdf" rel="nofollow noreferrer">http://research.microsoft.com/pubs/67977/ppdprelational.pdf</a></p>

<p>[The Wikipedia article gives only a vague definition and a long list of subfields.  Should just accept that there's no better definition than this? <a href="http://en.wikipedia.org/wiki/Theoretical_computer_science" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Theoretical_computer_science</a> ]</p>

<p>EDIT: I guess this question comes down to "What does the term 'theory' mean in the context of computer science?".  Looking at the 6 different meanings of the word at <a href="http://en.wiktionary.org/wiki/theory" rel="nofollow noreferrer">wiktionary</a>, I don't think any of them fully fits.  I guess the mathematical sense of a theory fits well for completely mathematical fields but not for others, and for VLSI, machine learning and computational biology from <a href="http://en.wikipedia.org/wiki/Theoretical_computer_science" rel="nofollow noreferrer">wikipedia:TCS</a> it basically doesn't fit.</p>