"Lifelong" is a descriptive word that means lasting for the entirety of one's life. There are several synonyms that can be used instead of "lifelong" to convey the same meaning. These include "eternal," "enduring," "perpetual," "everlasting," "continuous," "infinite," "unceasing," "undeniable," and "unlimited." Each of these words is intended to express the idea of something that lasts for a long duration or persists without end. Depending on the context, any of these synonyms can be used interchangeably with "lifelong" to provide a more nuanced description of something that is expected to endure for a long time.