In plain words
But there is another, the supreme Puruṣa, called Paramātma—the imperishable Lord who, entering the three worlds, sustains them.
Word for word
But the highest Puruṣa [is] other, called the supreme Self, who, entering the three worlds, sustains [them], the imperishable Lord.
उत्तमः पुरुषस्त्वन्यः परमात्मेत्युदाहृतःयो लोकत्रयमाविश्य बिभर्त्यव्यय ईश्वरः
Other commentaries
The commentators
▸Bhagavadgītābhāṣyam
आभ्यां क्षराक्षराभ्यां अन्यः विलक्षणः क्षराक्षरोपाधिद्वयदोषेण अस्पृष्टः नित्यशुद्धबुद्धमुक्तस्वभावः —
उत्तमः उत्कृष्टतमः पुरुषस्तु अन्यः अत्यन्तविलक्षणः आभ्यां परमात्मा इति परमश्च असौ देहाद्यविद्याकृतात्मभ्यः, आत्मा च सर्वभूतानां प्रत्यक्चेतनः, इत्यतः परमात्मा इति उदाहृतः उक्तः वेदान्तेषु । स एव विशिष्यते यः लोकत्रयं भूर्भुवःस्वराख्यं स्वकीयया चैतन्यबलशक्त्या आविश्य प्रविश्य बिभर्ति स्वरूपसद्भावमात्रेण बिभर्ति धारयति ; अव्ययः न अस्य व्ययः विद्यते इति अव्ययः । कः ? ईश्वरः सर्वज्ञः नारायणाख्यः ईशनशीलः ॥ १७ ॥
In plain words
There is another, utterly distinct from both the perishable and the imperishable, untouched by the flaws that come with either of those two limiting conditions — one whose very nature is eternal, pure, awake, and free —
'Uttama' means the highest. But there is another person, utterly distinct from those first two, called the Supreme Self, Paramātmā: 'supreme' because he surpasses every self that ignorance has fashioned, starting with the body, and 'Self' because he is the innermost consciousness of every being. That is why he is designated, in the texts of Vedānta, as the Supreme Self. And that very one is further described as the one who, having entered, having pervaded the three worlds, earth, sky, and heaven, by his own power of consciousness, sustains them, simply by the presence of his own true nature, holds them up. 'Imperishable' means there is no decay in him at all, that is what 'imperishable' means. Who is he? The Lord, all-knowing, called Nārāyaṇa, the one whose very nature is to rule.
Close to the Sanskrit
Different from these two, the perishable and the imperishable, other, of a distinct character, untouched by the fault of the pair of limiting adjuncts — perishable and imperishable — [his] nature ever pure, awake, and free —
'Uttama' means the highest; but the other person, utterly distinguished from these two, is called 'Paramātmā'; and he is 'parama' with respect to the selves made by ignorance, beginning with the body etc., and he is 'ātmā' because he is the inward consciousness of all beings; therefore he is designated, said to be, 'Paramātmā' in the Vedānta texts. That very one is further distinguished as he who, having entered, having pervaded, the triple world, called bhūr, bhuvaḥ, svar, by his own power of consciousness, sustains it, by the mere presence of his essential nature he sustains, holds up. 'Avyaya' means: there is no decay of him, thus 'avyaya.' Who? The Lord, omniscient, called Nārāyaṇa, whose nature is to rule. || 17 ||