An investor who has a holding of a particular security has a long position in it.
An investor with who has sold a security short has a short position in it.
More generally, a long position is one that will produce a profit from a rise in the price of a security, a short position is one that will produce a profit from a fall in the price of a security. This means that it is possible to have a long or short position indirectly by holding derivatives.
A long position in a call option implies a short position in the underlying asset.