Cyborg - The Commander has a big, presumably cybernetic eye - though it could also be a fancy monocle or eyepatch given that it seems to swap sides with each campaign. It glows red.
Death from Above - The Smartbomb powerup. It'll wipe out everything in a very large portion of the map. Using it five times nets you an achievement/medal called "The Only Way To Be Sure".
Difficulty Levels - Done in an interesting way. You start out on the hardest difficulty no matter what, and get rewards for sticking with it. If you lose that particular level, you can opt to keep on trying on the hardest difficulty, or generate an easier version. This usually reduces the number of enemies, points the spawn from, and often changes the map layout.
Difficulty Spike - The first time you see a Giant Titan, you'll likely only have the normal and heavy blasters, and you'll probably be low on funds to build them. It also shows up while other Titans are still filing in from all sides. It's about this time that you need to learn to sell off refineries when you're done with them and to get rid of turrets you don't need anymore. It's on the final level of Earth, by the way.
Frickin' Laser Beams - The Laser turret emits a short-lived, sweeping beam of energy. Whilst it has a short detection range, it can hit enemies outside the detection range, and absolutely murders Mega-Titans if it can engage them at short range and hit them with most of the beam. Unfortunately it's not very effective against heavily-armoured Titans.
Land Mine Goes Click - Mines, Cluster Mines, and Blast Mines. Mines are triggered if a Titan steps on them. Cluster Mines can be triggered up to four times, but aren't as strong as normal Mines. Blast Mines are manually triggered with a five-second countdown. With the blast radius and blast damage upgrades, they're pretty much a cheaper and more plentiful version of the Smartbomb powerup.
Mighty Glacier - Hear a roar? Check around the map for a Giant Titan. If it's glowing green, you better hope you've got a freeze power-up handy.
Mook Maker - A rare player-owned variant; the Robot and Tank factories slowly produce small automated combat units that can help put some extra pressure on the Titans, take some heat off your turrets, and target/engage micro-Titans. They get blown up if the Titans so much as step on them though.
More Dakka - The Multi-Blaster Turret, especially when flanked by multiple cooling towers, or when benefitting from the Bezerk power-up, which kicks all turrets into overdrive; instant reload, unlimited ammunition, maxed-out range and rate of fire.
Mundane Made Awesome - Most often, when you research something that allows you to research better turrets, the science advisor will tell you about all the scientific mumbo-jumbo, but will place a bit of emphasis on the fact that you can now make better textiles!
No Damage Run - Perfectly possible. You even get a $500 reward and medal for doing so. It says "The Titans didn't even scratch the paint."
Our Ghosts Are Different - The Effigia and Phasmatis Ghost Titans are first sighted near the end of the Mars campaign, and also appear later in the game. Without the X-Ray Scanner tech your turrets cannot target or hit them, they are barely visible if not near the Main Base itself. Fortunately you can use the Capacitor to zap them manually. With the X-Ray Scanner, turrets can spot and target the Ghost Titans, but do very little damage.
Pun-Based Title - The titles of most (if not all) of the Moon levels are wordplay to do with various types of cheese.
Shock and Awe - The Capacitor 'turret' dispenses streams of electricity on demand, but must be manually controlled. This does mean it can zap targets that the turrets cannot aim at, such as the mini-titans and ghost titans.
Tower Defense - The main focus of gameplay. Most of the Titans are at least moderately smart and will try to find weaknesses in your defenses - if you have many turrets bunched together on the roads, they might decide to try and cross an open field that isn't covered by turrets, even though it's slower. Some are smarter, and can even be duped into attacking other areas using the Decoy and Scarecrow - the former draws smarter Titans in with rhythmic thumping they find irresistable, whilst the latter scares away all but the most aggressive and determined Titans.
Unfriendly Fire - Explosives like Mines and Rockets can and will damage your own structures caught in the blast radius. The Rocket Turret at least has a minimum range, so it won't try and blow itself and its attendant defensive/utility structures up.
Unwinnable by Design - Sort of. Since your money and research carries over from one level to the next, you can make it very difficult to progress if you lose too much money (usually by losing turrets, building too many, and not mining enough) and/or spend all your research 'points' on donuts and theoretical techs instead of getting superior turrets. You can go back and redo earlier levels though.