Accident and Mistake
The defences of accident or mistake will have different meaning depending on the context.
Generally, an accident is a "mishap or untoward event not expected or designed", or "unforeseen contingency or occurrence".
Most typically accident means that the accused did not mean to perform the actus reus or that the consequences of the actus reus were unintended.
The effect in law is that the mens rea is not present. This is distinctive from mistake which occurs in “the realm of perception”.
Once the accused establishes there is an "air of reality" to the defence of accident or mistake, the crown must disprove the availability of the defence beyond a reasonable doubt. 
General or Specific Intent Offences
The meaning of accident varies depending on the type of charge. Where it is a specific intent offence, an accident relates to a denial of voluntariness of the act or denial of intention to cause the outcome. For a general intent offence, an accident requires that the act was unexpected and by chance that was not foreseeable. 
R v Whitehorne 2005 CanLII 34553 (NLPC)
R v Hill, 1973 CanLII 36,  2 SCR 402, (1974) 14 CCC (2d) 505 (SCC) at p. 510
R v Tatton, 2014 ONCA 273 (CanLII), at para 24
R v Mathisen, 2008 ONCA 747 (CanLII), 239 CCC (3d) 63, at para 70
- Whitehorne, supra
- R v Sutherland, 1993 CanLII 6614 (SK CA), (1994) 84 CCC (3d) 484 (Sask. C.A.) per Vanscise, J.A., at pp. 497-498
- see Intention#Specific and General Intent
Criminal Pleading & Practice, Ewaschuck, (2nd Edition) at para 21.0030
R v Mathisen, 2008 ONCA 747 (CanLII) at para 70