Why was 1942 the turning point of the war in Europe and the Pacific? What is meant by the term "turning point" (specifically related to German and Japanese aggression)?

In war, a "turning point" marks the period when the outcome of a conflict becomes more defined. In other words, the "turning point" provides some demonstration of which army will ultimately prevail. 
In WWII, two battles that mark turning points heralding the eventual Allied victory are:
1) Battle of Guadalcanal (Aug 1942-Feb 1943)
The Battle of Guadalcanal marked the first major offensive victory by Allied powers against the Axis power of Japan. After the Pearl Harbor attack, the Japanese moved swiftly to appropriate the Allied South Pacific route connecting Australia and the United States (July 1942). The Japanese army worked to accomplish this goal by setting up a base of operations on Guadalcanal in the Solomon Islands. By August 1942, the Japanese had 8,400 men stationed on the island; they also had an airfield and artillery positions in the hills.
What the Japanese didn't bargain for was the determination of U.S. Marines to retake this extremely strategic location from them. On August 7th of 1942, U.S. Marines landed on the island, and on Oct 13th, the U.S. Army provided reinforcements for the Marine effort. The Marines and infantrymen managed to wrest control of the island from the Japanese in about six months. The Japanese never recovered from its loss of air superiority after the Battle of Guadalcanal. Overall, the Japanese lost almost two thirds of its 31,400 deployed troops, while the Americans lost about 2,000 men out of a deployed force of 60,000.
2) Battle of Stalingrad (July 1942-Feb 1943)
The Battle of Stalingrad was fought to cut off the German advance into the Soviet Union. It was the one battle that caused a death blow to Hitler's dreams of world domination. The Battle of Stalingrad is perhaps the most significant battle in WWII and constituted a major turning point on the Eastern Front. The Soviet generals (Georgy Konstantinovich Zhukov, Alexandr Mikhailovich Vasilevsky, and Nikolay Nikolayevich Voronov) launched a modern-day Cannae offensive on the unsuspecting Germans.
The strategy involved picking off the weaker German flanks before penetrating deeper into the inner flanks. Hitler ordered the German Sixth Army under General Friedrich Paulus to stand and fight, despite the rise in German casualties. He maintained that the Luftwaffe would be able to resupply the Sixth Army with provisions and medical supplies, but Air Marshal Hermann Goring was unable to deliver on Hitler's promise. General Paulus' Sixth Army was eventually surrounded by seven Soviet armies.
The Germans never recovered their dominance after the Battle of Stalingrad. Total Axis casualties were estimated at around 800,000 men (150,000 of these were German casualties). When General Paulus surrendered, 91,000 German soldiers followed his example. However, of the 91,000 men, only about 5,000-6,000 managed to return home. The rest perished in Soviet labor camps.
https://www.britannica.com/event/Battle-of-Stalingrad