Question: What is the meaning of ‘Tathāgata’?
Answer: ‘Tathāgata’ is Sanskrit for ‘Thus Come One’ (如来), which means ‘as if came; also as if gone’. This is an alternative term for addressing Buddhas. The Buddhas as if came about, but did not really come, because what they came to realise was their ever-present Buddha-nature (i.e. potential for Buddhahood), which was never away from them.
After manifesting parinirvāṇa (or relinquishing of their physical forms) upon completing their teaching to many with direct karmic affinity, they are as if gone, but never really gone. This is so as Buddhas, being with perfect and endless compassion, never truly forsake any sentient beings, and will still manifest in various ways to guide them to Buddhahood.