(adjective)
occurring regularly or consistently, or persisting or recurring over a period of time;
e.g. I am involved in a constant struggle of convincing them to listen to me.
(adjective)
steady, firm or faithful, used to describe a person, their attitude, or their attachment to something;
e.g. She has been their constant supporter through everything.
(noun)
in mathematics or science, a numerical quantity or a parameter that remains the same under a set of conditions or that one does not change regardless of the circumstance;
e.g. This relationship is defined by a constant whose value is known.
(noun)
someone or something who/that is always available or readily present, or a situation that remains the same and doesn't change;
e.g. He has been a constant in my life through all of my struggles. The threat of being being found was a constant throughout their journey.