Ablation

Ablation is the removal of material from the surface of an object by vaporization, chipping, or other erosive processes.

Absolute Magnitude

The brightness an object would have if the observer was 1AU away from it.  This value is often used instead of the size of a NEO, as the albedo of the NEO is usually also not known.  Therefore, for most NEOs, it is not possible to determine the difference between a small bright reflective NEO and a large dull dark NEO, which would have the same Absolute Magnitude.
(For those that are interested, the Diameter of a NEO can be obtained using the following formula if the Absolute Magnitude (H) and Albedo (p_v) are known:
D=1329km×10^-H/5×p_v^-1/2)

Albedo

The ratio light reflected by an object compared to the amount of light that falls on it.  An albedo of 0 means the object absorbs all light and appears pitch black and 1 means it is a perfect reflector.

Aphelion

The farthest point of an orbit to the sun

Asteroid

Asteroids are small, airless rocky worlds that revolve around the sun and are too small to be called planets

AU

Astronomical Unit – the distance between the Earth and the Sun, 149,597,870.691 kilometres

Eccentricity

The difference between the perihelion & aphelion of an orbit.  The higher the difference, the higher the eccentricity of the orbit.

Inclination

The angle an orbit is inclined compared to the ecliptic

Keyhole

As an asteroid passes by a Moon or planet, the gravity of the object will deflect the path of the asteroid. The keyhole is space an asteroid must pass through so that it is deflected the exact amount to bring in on a collision course with Earth in a future orbit.

Mitigation

Reducing the severity of an outcome

MOID

Minimum Orbit Distance - the minimum distance between the two orbits (circles or ellipses) of two bodies.

Momentum

The measure of a moving body, calculated by taking the product of its mass and velocity.  It is also a measure of the impetus of an object and how difficult it will be to stop it.

NEA

Near Earth Asteroid

NEO

Near Earth Object – an object whose orbit brings it to within 1.3 times the distance between the earth and the sun, 1.3 AU or 200 million kilometres. The objects can either be comets that take less than 200 years to orbit the sun, or asteroids.

Palermo Scale

A scale rating the impact hazard of a NEO (see also Torino Scale for another measure), which compares the threat of a given NEO to the so-called background threat of all NEOs of the same size or larger between now and the predicted date of impact.  In this way, the probability of the impact itself as well as the time until the predicted impact are considered.
The scale is logarithmic, so a Palermo Scale of -2 is only 1% as likely as a random background event between now and the time of predicted impact.  A value of 0 indicates that the risk is the same as the risk from the background threats, and therefore NEOs with a Palermo Scale of greater than 0 are considered threatening.
(The Palermo Scale is defined in the paper "Quantifying the risk posed by potential Earth impacts" by Chesley et al. available here.)

Perihelion

The closest point of an orbit to the sun

PHA

Potentially Hazardous Asteroid – an asteroid that could make a threatening close approach to the Earth. In technical terms a PHA is defined as having an absolute magnitude of 22 or brighter and an Earth Minimum Orbit Intersection Distance (MOID) of less than 0.05 AU or 7.5 million km.

Semi-major axis

Half of the distance across an ellipse, measuring along its longest axis

Torino Scale

A scale rating the impact hazard of a NEO (see also Palermo Scale for another measure), which considers the impact energy as well as the probability of impact.  The scale is from zero to ten, and only applies to potential impacts in the next 100 years.
0: No Hazard (White Zone)
1: Normal (Green Zone) - very unlikely collision
2-4: Meriting Attention by Astronomers (Yellow Zone) - 1% or greater chance of impact
5-7: Threatening (Orange Zone) - uncertain but a threat
8-10: Certain Collisions (Red Zone) - certain impact, with different ratings for different damage zones