The term chazakah (Hebrew: חזקה — literally, "strong") usually refers to the default assumption; i.e., what is assumed until there is evidence to the contrary.
Also, one who engages in acts done only by kohanim is assumed to be a kohen himself, until proven otherwise (see Presumption of priestly descent).
[4] An article by R' Osher Weiss makes further distinctions between various types of de'oraita or derabbanan commandments, describing a hierarchy of no less than 18 levels of significance for mitzvot.
A kal vachomer (Hebrew: קל וחומר, literally "lenient and strict") derives one law from another through the following logic: If a case that is generally strict has a particular leniency, a case that is generally lenient will certainly have that leniency.
The argument can also work in reverse, and also in areas where lenient or strict might not be precisely applicable.