Athelas:

An herb brought from Númenor. In the rightful king's hands it acted as a cure for many ills. Aragorn used it in his attempt to heal Frodo of the wound caused by the Morgul-knife, though with little success. The dried leaves were more effective in the injuries sustained in the Mines of Moria however. Athelas was especially effective against the Black Shadow, as it affected Merry, Éowyn and Faramir. It did not grow wild in Middle-Earth, except in the areas where the Dúnedain had camped or lived.

Asea Aranion:

Athelas, as it is called in the Valinorean Tongue.

Arod:

A horse of Rohan. Arod is described as being a lighter horse and also a bit more hot blooded than the usual Rohirrim mount. This was the horse loaned to Legolas and Gimli.

Asfaloth:

The horse that Glorfindel was riding when he found Frodo, Sam, Merry, Pippin and Aragorn at the Last Bridge.

Alfirin:

This golden flower was known to grown in Lebennin.

The Ages of the Trees:

The ages that passed while the Two Trees were alive. They were only counted within Aman.

The Ages of the Stars:

These passed outside Aman, while the Trees lived, and after the Two Trees died, until the first rising of the Sun.

The Ages of the Lamps:

These lasted from the time that Almaren, the island that the Valar made their home, until the assault by Melkor, in which the Lamps were toppled. The Ages of the Lamps ended in fire and war.

The Ages of the Sun:

These lasted from the first rising of the Sun until present day. There were four Ages recorded:

The First Age of the Sun (The Elder Days)
The Second Age of the Sun (Sometimes counted in the Elder Days)
The Third Age of the Sun
The Fourth Age of the Sun (The Dominion of Men)

More information can be found under the individual entries for each of the Ages, under Events in Middle-Earth or Timeline for the History of Middle-Earth.

The Lord of the Rings
The Silmarillion
The Unfinished Tales
The Hobbit
The Complete Guide to Middle Earth by Robert Foster