Technical analysis is any method for generating trading signals, said method being based solely on past and present price data and/or volume data of a security.
This includes both quantifiable and unquantifiable methods, including mathematical formulae (RSI and ADX), trendlines, pattern recognition (wedges, triangles, flags, head-and-shoulders, etc.), candlestick interpretations, etc. TA is a broad collection of methods, some logic-based and others purely intuitional.
Note well: it is literally impossible that academia has tested all methods that qualify as TA and found them all wanting. Therefore academic studies claiming to "disprove TA" are bunk. All they can do is focus on some narrow aspect(s) of TA. Based on the studies I've seen, many studies are doomed to failure just on the basis of poor risk management (all-in trading is a frequent assumption among academics).