Riba

An Arabic word meaning: "increase" or "excess"

Tech: an 'increase' which in a loan transaction or in exchange of a commodity accrues to the owner (lender) without giving an equivalent counter-value or recompense ('iwad) in return to the other party; every 'increase' which is without an 'iwad or equal counter-value.

Detailed technical definition:

1) Riba is the contractual difference in the value of 2 or more items of the same type, quality and value when they are exchanged (irrespective of the time-period involved or the type, magnitude or form of the difference)

For instance: when, 1kg of grapes are exchanged for 1.5kg of grapes that are of the same type, quality and value - then the 0.5kg is riba. A more relevant example is: if $100 is exchanged for $110 - then the extra $10 is riba [in these examples; the nature of the exchange is what defines it as a sale, loan, etc. The first type of exchange results in a barter and the second results in a lending/borrowing transaction, the $10 being the 'interest']. 

Regarding time-periods; an exchange can be simultaneous or deferred - either way, if the items are the same then any differences in their exchange value is riba.

2) However, a contractual difference in the value of 2 or more items of different (dissimilar) type, quality and value when they are exchanged is not riba (irrespective of the time-frame or the type, magnitude or form of the difference) - if the contractual difference is fixed at the point of the transaction. 

For instance: when, 1kg of grapes are exchanged for 1.5kg of apples - then the 0.5kg (of apples) is not riba. Again, a more relevant example is: if 100 pens (value = $1 each) are exchanged for $110 - then the $10 is not riba [it is legitimate profit - i.e. mark-up].  Here, the second type of exchange results in a sale transaction.

3) However, if the agreed difference is changed post-transaction, then the amount of the 'change' is riba.

That is...if the 0.5kg of apples or the $10 is forcibly increased post-transaction to say, 0.8kg or $11, then this 'increase' becomes riba.

4) Classic (jahiliyah) riba is normally that which was charged by the lender for 'extra time to pay' or 'missed payments' when the borrower could not pay the loan within the original time period or within the agreed payment mode. This was expanded by the messenger of God, Mohammed (pbuh) later to include the above definitions - and further expanded by the scholars (fuqu'ha) to include what is known as Riba An-Nasi'ah (differences in time) - which is:

...forced delays in simultaneous exchanges of 2 or more items of the same (similar) type, quality and value - post-transaction (irrespective of the magnitude of the delay).

This is where say, an agreement is reached between 2 parties whereby 1kg of grapes of a particular type, quality and value owned by party A, are to be simultaneously exchanged for 1kg of grapes of the same type, quality and value owned by party B - but rather than carrying out the exchange in real-time (spot), one party unjustifiably delays the exchange (which could potentially put the other party at a disadvantage). 

There is no real proof (from the Qur'an or sahih hadith that the above 'differences in time' is actually riba - just that the messenger of God, Mohammed (pbuh) stated that 'like for like' exchanges are to be carried out (and be paid for) in real-time.

"Gold is to be paid for by gold, silver by silver, wheat by wheat, barley by barley, dates by dates, and salt by salt - like for like, equal for equal, payment being made on the spot. If the species differ, sell as you wish provided that payment is made on the spot" - Sahih Muslim

In conclusion: riba is not solely applicable to monetary exchanges or loans as it (under specific circumstances) is also applicable to barter/trade transactions as well. If there is a difference between traditional understanding of the English word "interest" and Riba - then it is here...as normally, 'interest' is almost always applicable in monetary exchanges, whereas of course, riba is not. In other words - interest is riba, but riba is not restricted to just 'interest'.

Definition of the English word 'interest' in relation to financial transactions and riba:

The English word 'interest' (in financial transactions) refers to a surcharge on the repayment of debt (borrowed money), rent paid for the use of money, the fee charged by a lender to a borrower for the use of borrowed money, etc.

Thus, interest is:

A monetary charge applied for the use of money

...and since this charge is an 'increase' in a like for like exchange (money for money) - it is riba and thus forbidden in Islamic law (Shari'ah)

Gharar

The Arabic word gharar does not have a universally accepted defintion. However, its general meaning is: unqualified & unquantified uncertainty, hazard, deceit, chance or risk (khatar).

Tech: the element of risk that is unpredictable and has no measurable probability; sale of a thing which is not present at hand; or the sale of a thing whose aqibah (consequence, outcome) is not known; or a sale involving risk or hazard in which one does not know whether it will come to be or not, such as a fish in the water or a bird in the air.

In modern terms - it can refer to a sale of an item on purposefully unclear and ambiguous terms (e.g. sale of a house without giving full details of its structural integrity, how many rooms it has, the size of its boundary, etc).

While the prohibition of riba is absolute, some degree of gharar or uncertainty is acceptable in the Islamic framework. Only conditions of excessive gharar need be avoided

Maysir

An ancient Arabian game of chance played with arrows without heads and feathering, for stakes of slaughtered and quartered camels.

Tech: it came to be identified for all types of games of chance - i.e. gambling (the Islamic definition of which is: a game of chance where there is a material loss for the loser )

Also referred to as Qimar

Zulm

A comprehensive Arabic term used to refer to all forms of inequity, injustice, exploitation, oppression and wrong doing, whereby a person either deprives others of their rights or does not fulfil his obligations towards them.

Also refers to trading in items the use of which is prohibited in Islamic law (Shari'ah) such as intoxicants (e.g. alcoholic drinks & illegal recreational drugs), pork, etc