Determination

Determination is a power that allows SOULs to persist after death. The most determined being in the Underground is able to SAVE.

Overview
Determination is naturally possessed by humans. Should a monster acquire a high concentration of determination, its body melts, usually into an amalgamate. The only known monster who may naturally possess determination is Undyne. A being without a SOUL can gain the will to live if it acquires determination, a prime example being Flowey.

Physical Determination
The game never makes clear how physical determination would look like after extracted, if it's liquid, solid, vapor or if its visual form resembles a magic spell of some sort. On MENUs and SAVE Stars, determination is represented by the YELLOW HIGHLIGHT, but cannot be assumed that physical determination is yellow, considered something Fan-canon, making this subject of determination's physical appearance, fall on the Fudge category on RIFTALE and its Encyclopedia.

Main Story
Save Points can be found throughout the game. These SAVE points express the protagonist's determination and thoughts in regards to recent events.

True Pacifist Route
In the True Lab, it is revealed that Alphys started experimenting with determination extracted from human SOULs in an attempt to unleash the power of the SOUL. She used blueprints to operate the DT Extraction Machine to obtain determination and began injecting it into the bodies of monsters who had fallen down. However, since their bodies cannot handle determination, the monsters injected with determination soon melted together and became Amalgamates.

Genocide Route
Undyne, fueled by her determination, challenges the protagonist as Undyne the Undying. When defeated, she melts in a similar fashion to Amalgamates like when she dies in her normal form.

At the end of the route, the fallen human speaks to the player directly, explaining that they were brought back to life due to the player's determination.

Trivia

 * The SAVE point description in Temmie Village refers determination as "detemmienation."
 * "Determination" might be a play on words, as the primary power it affords is the ability to reverse death, either the protagonist's or those they may have killed, hence de-termination.
 * It is unknown if the Amalgamates mimicking game elements such as Frisk's "!" dialogue box or SAVE points are actually happening and related to the Determination they have been injected with, or are simply breaks of the fourth wall intended to catch players off-guard.
 * The DT Extraction Machine resembles Omega Flowey.