Explain what is meant by aggression.

In terms of psychology, at least, aggression is defined as
any act that is meant to bring harm to another person--to make them suffer or to in some
way damage them.  This means that aggression can take many
forms.

For example, there can be direct physical
aggression.  Punching someone is meant to hurt them and is therefore aggression. 
Stealing from someone's home could be aggressive as the loss of valuable possessions
would cause emotional pain to the person.  Finally, words can be aggressive.  We know
that words have the power to hurt.

Additionally, the link
below give detailed discussions of aggression and the causes of
aggression.