Here are some words that beginners may not know about, but are essential when discussing openings. If anyone has any words to add, please do so:

Gambit:

This is when material is sacraficed for something else, for example:

A lead in development

More active pieces

Capitalizing on a weak move of the opponents

better pawn structure

and so on.

Open games:

Chess games starting with 1. e4 e5

Semi- Open games:

Chess games starting with 1. e4, and something other than e5. The most popluar is c5, the Sicilian Defense.

Closed games:

games starting with 1. d4 d5

Indian Defenses

Games starting with 1. d4 Nf6

Flank openings:

Opening moves by white other than e4 or d4. Most notably the English (1. c4) and 1. Nf3.

I

Also:

Countergambit

A gambit by Black, although not all gambits by Black are countergambits. It does not need to be in response to a gambit by White. For example:

Nice example.

Thanks rgp89.

Also, note that not every gambit by black is called a countergambit. For example:

This is the Lavitan Gambit:

1. e4   e5

2. Nf3  f5

This is a gambit by Black, but not called a countergambit.

Another word:

Indian Defense:

Any opening system that begins 1. d4  Nf6.

Examples include Nimzo Indian Defense, Bogo Indian Defense, Queen's Indian Defense, King's Indian Defense, and Dzindzi-Indian Defense.

The Latvian Gambit is also called the Greco Counter-gambit.

The Lativan Gambit was FORMERLY known as the Greco Countergambit. The name "Lativan Gambit" is now the official name. (Thanks for pointing that out anyway BILL_5666!)

 

It's not the only opening to have more than 1 name (Ruy Lopez/Spanish).  Having an "official" name does not invalidate an older name.