Historically, Technical SEO (Search Engine Optimization) has included work by specialists dealing with client and server-side technologies that focus on search engine crawling, rendering, and indexing.
Merkle Defines Technical SEO as:
Technical SEO is defined by configurations that can be implemented to the website and server (e.g. page elements, HTTP header responses, XML Sitemaps, redirects, meta data, etc.). Technical SEO work has either a direct or indirect impact on search engine crawling, indexing and ultimately ranking. As such, Technical SEO doesn't include analytics, keyword research, backlink profile development or social media strategies. (source)
Any sufficiently technical action undertaken with the intent to improve search results.