तितिक्षा
titikṣā
Bearing passing contacts such as heat and cold, pleasure and pain, without running from them or freezing; the practice of staying.
Titikṣā (तितिक्षा) is the capacity to bear that appears in the command titikṣasva in verse 2.14: the contacts of the senses bring cold and heat, pleasure and pain; they come and go and are impermanent; so bear them.
Note that the Gita does not use the noun; verse 2.14 has only the verb titikṣasva. The name "titikṣā" is the tradition's label, which captures the gist of that command, so read it as the tradition's name for it and not as a word the Gita itself uses in this form.
It does not mean gritting the teeth, and it does not mean pushing the experience down. Suppression turns away from experience; titikṣā turns toward it and stays. The ground is the one the verse gives: it comes and goes.
Titikṣā is the first step toward equanimity. Staying with it builds the steadiness that verse 2.15 describes as not being disturbed.
For more, see the topic on equanimity.