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LC50  (Lethal Concentration, 50%) -- A point estimate of the concentration of an effluent (or toxicant) at which 50% of the test organisms will die within a specified period of time (usually 24-hours, 48-hours, or 96-hours). The LC50 is usually an endpoint for acute toxicity tests. 
     For example, if a test yields an estimated 24-hour LC50 of 70%, that means that 50% of the test organisms would be expected to die within 24 hours in a 70% concentration of the effluent.

NOEC (No Observed Effect Concentration) -- The highest concentration of an effluent or toxicant which causes no observable adverse effects on the test organisms (i.e., the value for the oberved responses are not statistically significantly different from the controls).  This is an important endpoint for both acute and chronic definitive tests.
     For example, if the results of a test indicate a NOEC of   25%, that means that the 25% concentration of the effluent was the highest concentration at which no adverse responses were observed that were significantly different from the controls.

LOEC (Lowest Observed Effect Concentration) -- The lowest concentration of an effluent (or toxicant) which causes observable adverse effects on the test organisms (i.e., the observed effects are statistically significantly different from the controls).  This is an important endpoint in both acute and chronic definitive tests.
    For example, a LOEC of 50% means that adverse effects caused by the effluent would be observed at effluent concentrations of 50% and higher.

IC (Inhibition Concentration)   -- A point estimate of the effluent (or toxicant) concentration that will cause a given percent reduction in a biological measurement such as reproduction or growth.   This is a required endpoint in certain chronic tests.
      For example, an estimated IC50 of 65% for growth means that growth of the test organisms would be expected to be reduced by 50% when the test organisms are held in a 65% concentration of the effluent or toxicant.   An estimated IC25 of 70% for fecundity means that reproduction of the test organisms would be expected to be reduced by 25% at an effluent concentration of 70%.  

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