I've been reading a lot about individual battles fought by US troops during WWII and come across numerous references to reinforced units at pretty much every level from company through division and even a few corps, often with (Reinforced) after the unit name. It seems to occur most often in the Pacific and Marine units, especially are rarely listed without the qualifier. I had assumed they were just over-strength units meant to absorb casualties, but now I'm wondering if they weren't just units that had assets from a higher organization attached. What are these referring to specifically?
You are correct that reinforced units had extra assets attached, before making an attack on an important target. In the US Army, every Corps and Army had a pool of independent Armor, Artillery, Engineer, Recon and Tank Destroyer battalions. These battalions could be plugged into a division making a major objective attack. For example, the US Army XV Corps only had one infantry and one armored division. It also had eight battalions of artillery, five battalions of armor, five battalions of tank destroyers, three battalions of Recon and six battalions of engineers.
Source: "Panzer Leader: Rules, Designer's Notes and Campaign Data" by Dave Clark and Nick Smith (1974)
Yes, WWII saw many reinforced and adhoc "tactical groups" and "task forces."
At first during WWII, these groups were of not a standard size, and a task force could be anything from a reinforced company to a reinforced battalion. At first, they were initially only formed to achieve an often immediate, limited objective. A good example of this would be Task Force Raff; a unit comprised of Glider-borne Infantry and Artillery, Tanks from an independent tank battalion and reconnaissance from the 4th Cavalry Squadron. The purpose of this task force was to penetrate German lines and establish contact with landed paratroopers, as well as establish supply lines. By the end of D+2 the Task Force had accomplished its objective and disbanded.
However, by the tail-end of the Battle for Normandy, the idea of semi-permanent taskforces within divisions begins to take hold, with the (correct) assumption that this would improve co-operation and cohesion. Infantry divisions often established far more robust Regimental Combat Teams, which often had a consistent company of engineers and a battery of artillery and perhaps a platoon from the divisional Cavalry Troop. Armored divisions, especially the light ones, began to form Battalion-sized Task forces that often consisted of 2 Companies of Tanks, 1 Company of Armored Infantry (or vice-versa), a battery of artillery, a tank destroyer platoon and a platoon of reconnaissance. Several such battalion sized task forces would form the maneuver elements of a brigade-sized Combat Command. These semi-permanent organizational changes also allowed for the maneuver units of a division to be far more flexible; with fire support being under the immediate control and within contact of the Regimental and even Battalion commanders.
The 4th Armored division is an excellent example of how semi-permanent unit attachments can greatly increase the co-operation between organic and non-organic units. The 704th Tank Destroyer Battalion, ostensibly not part of the 4th Armored division, became a de-facto permanent attachment, with very few breaks. By the end of the war, the surviving and new officers and men were therefore very intimate with their counterparts.
The Germans and Commonwealth formed similar taskforces of varying sizes, called "Kampfgruppes" and Battlegroups. They operated with all the same effectiveness, or lack thereof as the other belligerents. The most successful of these groups were formed with enough time for the units to become familiar with one another, and perhaps even conduct limited exercises just behind the lines. German KGs formed of second-rate neighboring (re: Units somewhat familiar with one another) units were especially effective in Market Garden in delaying the British Airborne forces until effective counter-attacks could be organized.
In modern terminology for the US Army, "Task Forces" now describe Battalion sized combined arms units; whereas "Company Teams" describe a reinforced/combined-arms units smaller than a Battalion. Several Task Forces form a Brigade Combat Team - showing a clear link to the lessons learned in WWII and the wars immediately thereafter.