The theory or conception of guru in various strands of Buddhism may be called “Buddhist gurulogy.” The primary meaning of the Sanskrit word guru is “heavy” as opposed to laghu “light.” By extension, a spiritual mentor would be called a guru. From a Buddhist perspective, a guru, like a buddha, would be a “field” (kṣetra: zhing). That is, one may “sow” beneficent or maleficent “seeds” in the field and accordingly “reap” rich yields either in the form of puṇya (bsod bsams) or pāpa (sdig pa). Thus, a guru may be seen as a “person of heavy/grave consequences.” [Add many points here.]
Th Bodhisattvabhūmi (Dutt, p. 75): dharmaguravo hi buddhabodhisattvāḥ | dharme hi tatkriyamāṇe pareṣām adhimātraṃ dharmagauravam utpadyate|; Tib. (Gangs can rig brgya’i sgo ’byed lde mig 24, p. 123): chos ni sangs rgyas dang byang chub sems dpa’ rnams kyi bla ma yin pa’i phyir te | chos la bkur sti byas na gzhan dag kyang shas cher chos la gus pa bskyed par ’gyur la |. Paraphrase: The dharma is the guru of buddhas and bodhisattvas, and hence by according respect to dharma, one would also generate respect for buddhas and bodhisattvas. In other words, if one respects dharmaratna, one would also come to respect buddharatna and saṃgharatna.