When is search said to be illegal? Is evidence collected in the course of an illegal search admissible in trial proceedings?

Search without a valid search warrant (i.e. issued
by other than a competent authority or without a search
warrant) results in an illegal search without authority of
law. However, due to this reason, the accused cannot get
benefit. Accordingly, evidence collected even during an
illegal search and seizure is considered admissible in trial
and adjudication proceedings.