What is investment?

An investment is
a type of transaction.

In a transaction,
something
is exchanged for
something else.  

By adding the concept of time,
we split the concept of transaction in two:
barter transactions
and
investment transactions.

In a barter transaction,
something (in the present)
is exchanged for
something else (in the present). 

In an investment transaction,
something (in the present)
is exchanged for
something else (in the future).  

And since present is certain –
and
the future is uncertain –
the concept time adds risk
to investment.