I learned many meaning of vitaka.. Like initial thought, or thought seed or directing mind toward object.. Whereas vicara is continuity of thinking about that object.
Similarly nimitta is arising of some mental sign/clue etc. Like I remember today's event, or I heard something etc. There is sanna also in conjunction with nimitta...
I want to know whether vitaka and nimitta same or different? If different I want to know practical understanding of the difference between them ?
Our thoughts follow a chain of associations, one after another after another. Assuming you're not a robot you can insert a particular idea into the chain, at will. It's as if you say to yourself: "let's consider the sun" - and as you say it you make an effort to remember/imagine the sun's characteristic signs (nimitta). As you keep your attention circling on those signs, your mind assembles them together into an idea of sun. This idea of "sun" is not much more than a label your mind gives to the combination of the characteristic signs, plus a little bit of samsara magic. The act of mentally assembling the signs into an object is called manasikara. Then, as you keep performing manasikara, in other words as you keep fabricating your idea of sun, the memory starts coming up with associations - various relevant ideas about the sun. You remember that it's a source of light and warmth for our planet, or maybe you remember the solar flares and the shape of the heliosphere.
The initial act of inserting the sun into your chain of thoughts (or turning your mind to it) is the vitakka, and the entire sequence of primary and secondary associations that follow a vitakka is what's called the vicara.
Nimitta are the characteristic signs (of the sun, e.g. the roundness, the yellowness, the hotness, the hugeness, etc) that your mind uses to assemble an object. For example as you try to visualize the sun you may imagine the feeling of warmth or burning on your skin. That imaginary feeling is one of your nimittas for the sun. Everyone uses slightly different nimittas for the same concepts, because everyone's memories and experiences of things are different.
Manasikara is making those signs into an object. For example a male's eye catches a sign of lady's curves. His mind grasps onto this sign, plus a couple more signs like the long hair moving rhythmically as she walks, the thinness of ankle bone emphasized by her sandals, pitch of her voice, - and assembles those into a particular cohesive image of a lively, bright, optimistic, and sexually attractive woman. That's manasikara. In regular life it happens subconsciously and automatically, whether you want it or not, at least until you learn to control it.
Once manasikara has happened there's always a chain of associations that follow the object, whether they're wholesome or not. Vitakka is when this process is kicked off artificially and deliberately.
If you keep the mind going down the chain of associations it will eventually follow some tertiary association and venture off the sun and onto all kinds of other topics, like ecology and politics. At that point you may have to do another vitakka by turning your mind to whatever nimittas of sun that you can think of. This restarts the process with a new vicara which may end up going in a different direction than the first one.
This is what you do in (basic) meditation, you pick an object (a focal point, a theme, technically a sign or set of signs) and repeatedly "strike the bell" by doing a vitakka, then "let the bell ring" by allowing a vicara to unfold. You repeat this cycle as many times as you need to achieve the objective of your meditation - generating disgust, or joy, or whatever it is you are after at this stage.
MN 20 is a good sutta where both terms nimitta and vitakka are used, and you can see where they overlap. https://lucid24.org/mn/mn020/index.html
nimitta is more general purpose, lower on the raw sensory food chain, than vitakka. Vitakka is verbal thinking, for example, mentally saying to yourself, "am I happy or angry." (not speaking that verbal thought out loud).
But one doesn't have to mentally talk to themselves to know if they're angry or happy. The subverbal knowledge of that, in sutta terminology, would be paying attention (manasi karoti) to perceptions (sañña).
A Nimitta could be a subverbal perception, or as in the case of MN 20, it's referring to either subverbal perceptions or the verbal thought chain that follows the subverbal impulse.
As you said, I think of vitakka as "directing the mind" (towards vicara). I suppose there's an element of "intention".
Conversely I think of "nimitta" as being the "sign" -- a superficial characteristic -- by which something is recognised. If I try to remember what my Dad looked like, for example, I might remember a beard, the kind of clothes he wears, the way he stands -- and if I saw that (beard, clothes, posture) in the street then I might recognise, "that's my dad!" ... and I think that's what the "signs" means.
One of the "nimitta" in a human being might be secondary sex characteristics -- long hair or a beard, breasts or wide shoulders, a skirt or trousers.
I think the suttas might encourage people not to get caught up in (not to grasp or attach to) "signs", for example AN 4.37
When a mendicant sees a sight with their eyes, they don’t get caught up in the features and details.
Idha, bhikkhave, bhikkhu cakkhunā rūpaṁ disvā na nimittaggāhī hoti nānubyañjanaggāhī.
